The AI in Fire Emblem games tend to have very poor decision-making skills. Archers will often go straight after mages, which are often the only units (other than other archers) that can counterattack from that distance, and they may even do this if they need to be one square from your Lord to do so. Other enemies will see a line of five soldiers and will all choose to swarm either the one riding a dragon who kills things in one hit or the heavily armored one who doesn't take damage. And the bosses have this weird concept that the best way to fight the heroes is to stand perfectly still in their room until you're within range, even while the heroes start filing in around them. In most cases, unless they are scripted, the AI will most of the time choose to attack any units that gets into their attack range, even when they are going to do no damage on the target (like some cavalier with a wimpy weapon attacking a knight, or a mage attacking a high-res bishop).
Further expounding on the boss problem, in the early section of the game, a boss may have only weapons with a single square of range, making killing them with archers and mages a cakewalk. Later on, you'll start seeing magic bosses, and even the physical-weapon-wielding bosses will get throwing weapons, which have a two-space range, but players can get longbows, which have three range, again making it easy to kill bosses with archers.
The whole suicidal attack on the mages thing is actually explainable. The AI is aiming to cause anyfatalities it can, no matter the cost to its own units, and mages tend to be... rather bad at taking physical damage note on the flip side, mages tend to be pretty good at resisting magic attacks; physical units tend towards the inverse (i.e. good physical defense, poor magic resistance). Keep in mind that winning a match in Fire Emblem is quite easy — winning with everyone alive is the rub. Given a 1% chance of an instant kill, and a 100% chance of doing half of the max HP of the target, the AI will go for the 1% instant kill chance every time and hope it gets lucky just to spite you. Most of the time, you will laugh at the foolishness of the computer, but when its attack successfully connects...
To be more precise (at least in the GBA ones), the AI first targets any units it can deliver enough damage to kill in one hit, regardless of the likelihood to hit or the damage that it will receive, and if it can't kill anyone, then it goes for the one that it can deal the most damage to, again not counting likelihood to hit. The issue is that the people with the lowest defense are generally your magic users. They also happen to have ungodly dodge most of the time (at least for Sages), and are fully capable of one-shoting most units in the game once they've been trained. The GBA games do not take into account whether they'll be counter-attacked at all, leading to sending a Sage out in front of your army and letting him destroy half the units on the map being a legitimate strategy. And because of the way probability to hit works in this game, a 30% chance to hit is really more like a 10%, and that's the sort of hit rate opponents often get.
The stationary thing applies mostly to bosses who are on seize points, and if they were to move, then you could rush right up and seize the gate/throne.
In an early stage in Radiant Dawn, you are forced into using only two units: the Squishy Wizard Micaiah and the Black Knight. Since the enemies are there specifically to kill Micaiah and the Black Knight is there specifically to protect her, you'd think they'd send the entirety of their oddly large force at her at once. Nope, they go two or three at a time and hack at the first living thing they see.
Even more fun is that enemies will always, always go straight for an unarmed unit, as they (obviously) can't counterattack (or if laguz or staff-users, not very well). You can easily get Micaiah to level 20 without a problem on that map by parking the Black Knight somewhere with his sword unequipped and letting the enemies flail pointlessly at him while Micaiah whittles them down from afar.
This has applications beyond power-leveling. Many Fire Emblem veterans know that the best way to save a mission that's going pear-shaped is to unequip your strongest character's weapon. Picture this: the enemy has three Swordmasters standing next to a mission-sensitive character, who has only a few HP left. In order to win the map, the enemy need only attack with a single unit. Yet if you move an armored unit up and unequip their weapon, any enemy unit within range will immediately abandon their attack on the almost-dead Lord and attack the armored unit instead, even if they can't damage it.
This is nicely averted with bosses who seem to stand still on the throne. So you carelessly move your Squishy Wizard up, planning to attack next round, when the boss runs right up and kills your exposed wimps. Ooo-ps.
Then again, this makes it possible to slip past them and seize the throne for an automatic win by sending a disposable unit into their attack range at low HP. On some maps, it can be worth spending a deployment slot on an untrained rookie to have them serve as bait in this way.
Which extends to neutral/allied units as well. Particularly noticeable in 3-10, where, for example, the leader of the Crimean Knights, Geoffrey, will move right up to a Bishop and then not attack them, but then Astrid will take a shot with her longbow. (Thankfully, the longbow is very inaccurate, as this was one of those situations described below where the enemy was being spared for thieving purposes.)
Neutral units fall into this, especially if they're the ones you have to talk to and recruit. For instance, when Erk shows up in the second story arc of Blazing Sword, he comes out of a village and polishes off two mounted units before you get to him. If you don't get to him immediately thereafter, though, he'll run off and provoke a boss with high Resistance and physical attack.
They also have a tendency to attack enemies that you don't want attacked just yet (usually because they have an item that can be stolen), throw themselves at enemies far more powerful than they are, and always make their moves in the exact same order, which means that on the rare occasions that one of them does have some sense of self-preservation, they'll often retreat when it's really not necessary. And they always retreat when it's time to heal, even if they're using a healing item. Even if it means leaving a plot-sensitive character exposed.
The Radiant Dawn AI also prioritizes units who have "rescued" someone. In theory, this makes sense, since if you rescue someone, the rescuer loses half Skill and Speed, making them sitting ducks who can't hit the broad side of a barn. Except tanky units don't care (at least for survival), and more importantly, they still do that if the rescuer has the skill that nullifies the stat penalties, meaning Tibarn (who starts strong and has that skill) can easily make all enemies flock to him and kill them on counterattacks, just by grabbing someone. Who needs Provoke?
The Shadows of Valentia has an AI that will pull units back to heal if their health gets low, which makes sense on its own. Then you realize that ONLY the people near death retreat to healing spots, and they do it without attacking anyone in their way (even if they have no fear of counterattack). Finally, the AI sends EVERYONE near death, often not accounting for the limited healing spaces if they don't have healers, meaning they send near death people to heal, then leave that one behind while the rest start to head back to fight. This makes it stupidly easy to cut the enemy army in half, even if it's mostly temporary. A good way to take advantage of this is to have an archer or two sit in the path between the enemy army and a healing spot and let loose. (Keep in mind that this doesn't affect summoned units, which the AI just summons more of anyway.)
The AI also has a tendency to ignore your equipment. Meaning that flying units will attack Mages with wind magic and armored units will attack Swordmasters (who, like Mages, have low defense and high avoid) with Armorslayers.
Then you get the units who are so outclassed by every player unit in range that it's literally impossible for them to actually do any damage (because they have zero chances to hit or no expected damage — or both) but insist on attacking anyway.
Enemies also have absolutely no experience with such advanced concepts as "strategic retreat", "regrouping", or "mixed unit tactics" beyond, as stated above, going after the most vulnerable unit available. However, this is circumvented by good level design and the player's general mindset... Usually. Having a single little soldier charge blindly at you when his allies are still too far away to assist is an unfortunately common occurrence.
The AI in Path of Radiance seems to prioritize doing damage/lowering their own damage taken above actually killing your characters — for example, putting Nasir in range of Ashnard will cause him to attack Nasir even if he could kill Ike with his next hit.
Ashnard will also attack Nasir at melee range, even though his weapon can attack at range (and Nasir then can't counterattack).
In Awakening, NPC characters are even brain-deader than usual for the series. One mission, involving the players escorting a group of villagers, will have said villagers running straight into the enemy's waiting swords when they would be safe if they just stayed put or tried to keep out of the attack range. The villagers in this mission are actually following a fixed route towards the bottom corner of the map (the prior cutscene establishes they're fleeing in a blind panic) and will escape if they get there, but since that's going right through a thicket of enemies and the mission is usually over before they get close, it makes them seem like lemmings.
The AI in Awakening also pays no attention to a player unit's pair-up partner. They will gladly rush towards that unarmed Troubadour, completely oblivious to the fact she'll be replaced by a General by the time they get there.
Fates includes a similar bunch of actively suicidal villagers in Sophie's paralogue. This time, they start in a perfectly safe corner and will actively run towards the nearest enemy.
The AI in Fire Emblem Fates generally understands the pair-up mechanics pretty well, barring the fact it will never dynamically create or split support pairs (let's face it, the game would be even more unreasonably Nintendo Hard if it did). It will even switch a paired unit around to get an advantage. However, it shows no foresight in doing so, and will only switch for immediate benefit, and not to allow the flying back partner to completely flank the player.
For a more specific example, see Hans in Chapter 3. He swiftly establishes himself as a piece of Dumb Muscle by charging an entrenched enemy position and getting his ass kicked. Fortunately, there's no gameplay reward for saving him, and he certainly deserves it after starting the fight completely unprovoked and in direct defiance of the Avatar's orders.
Then there's the behavior fans have termed "Matthis Syndrome", which involves recruitable enemies attacking the very characters who can recruit them. This often leads to absolutley ridiculous cases of Gameplay and Story Segregation, most notably with the character it's named after, who will willingly attack, and most likely kill his sister Lena, all the while calling out for her in his battle quote. He's the most famous example, but there are others, such as Astram having no qualms about attacking his lover Midia or Wendell openly attacking you despite saying he has "no love for war". Granted, some enemies in the series are smart enough not to attack their friends/relatives/lovers, but it's impossible to know which without risking it.
And sometimes units who can recruit themselves by talking to a certain character will prioritize doing so over staying alive. Palla and Catria are a good example: the first thing they do after spawning as reinforcements is fly over to Marth and talk to him. The problem is that the map where this happens is full of archers, and Palla and Catria are the first units to move on the enemy phase. This means that, if Marth is standing in range of an archer, they will fly to Marth, recruit themselves, and then immediately get shot and killed, before the player even has chance to control them.
Chapter 2 of Genealogy of the Holy War has you dealing with Princess Raquesis's three mediocre Paladin bodyguards who are very suicidal when there are enemies in their ranges. These knights will surround their princess by default, so if you want to play defensively or don't want to risk having them killed off in order to obtain the very valuable Knight Ring you're given if the three and their Princess survive, you have to keep Raquesis miles away from the incoming enemies.
In Fire Emblem Awakening, there's a paralogue where you recruit Action Girl Severa. Her paralogue has a trick: she won't automatically join your group when either Chrom or Severa's future mother Cordelia are sent to talk to her, but you have to escort her to another spot in the map so she can talk to an NPC Villager she had befriended. (You better not kill that Villager, doing so requires facing Severa's wrath to the point of a Face–Heel Turn!) Her poor AI causes more trouble in such a mission, as she tends to attack nearby enemy units to the point of dying... or sticking to the northern wall where the Tricksters can nearly one-shot her with their Levin Swords.
The cardinal rules for Fire Emblem AI are simple: 1. Absent being scripted to stay put, a unit will attack if an enemy is in range even if it will both inflict zero damage and die on the counter. 2. The AI hates you, the player, specifically, and will attempt to kill characters over actually completing its theoretical objectives.
And then the Conquest route of Fates averts rule number 2, and shows why Tropes Are Not Bad: the enemy's goal is to cross a line on the map, the AI knows this, and it seems to have at least one turn of lookahead - if it's a choice between attacking Corrin or moving out of range so it can reach the objective next turn, it will ignore 12 games of precedent and ignore combat odds with Corrin for the chance of winning the map outright. It's not much, but it's a startling Difficulty Spike for the AI to think ahead like that.
Delthea is a great example of this. She must survive the chapter in order to be recruited, but it's very hard to predict what she'll do with her Teleportation skill. Sometimes she'll wait until you get close enough to the boss to attack, sometimes she'll teleport on her very first turn and attack, and she'll even attack her own brother, Luthier, (making her a case of the aforementioned "Matthis Syndrome") who can counterattack and possibly kill her.
While most of the examples of stupidity in Fire Emblem Heroes are typical to the series, the skill system means that a new case is added: assist skill loops. Units who aren't in range of enemies will prioritize using assist skills over anything else, meaning that it's possible for the AI to get caught in loops of doing nothing but using assist skills on each other while ignoring the player's units. For example, a unit might use Pivot to jump over their ally each turn, only for that ally to use Reposition to bring them back to their starting square, and then keep doing this until the player brings their units into range.