There's a reason fandom refers to Mage Wardens as Easy Mode.
The Arcane Warrior is pretty much totally invincible once you have Shimmering Shield. You barely need a party on the hardest difficulty with this spec, even the hardest bosses can be easily worn down by yourself.
The bug with Shimmering Shield not draining mana like it was supposed to was patched, making Arcane Warrior tanks a little less ridiculous.
The Mage spell Mana Clash drains enemy demon, abomination, or spellcasters' mana pools, and does Spirit elemental damage (which few enemies have a resistance to) in proportion to the mana it drains. It is entirely possible to one shot even the strongest of enemy mages with this spell, especially when empowered by its prerequisite Spell Might. It can usually take out entire rooms full of opponents in a single shot, which basically allows you to walk through the Circle Tower quest (where about 90% of your opponents will be vulnerable to it). It also does more damage to the victim if they have larger mana reserves, which means it allows you to make mincemeat of magic-wielding bosses (sometimes you can even kill them with a single shot if you use the aforementioned Spell Might). The game itself seems to agree about how grossly unfair this spell is, seeing as enemy mages will never cast it on your party. A lot of missions in the game become much, much easier if you bring along a Mage who can cast Mana Clash.
Crushing Prison stuns enemies while dealing damage (and you're free to deal plenty more from your next attack).
The room-clearing Storm of the Century (a combination of AOE's Tempest and Blizzard)
Combine Tempest, Inferno, Blizzard and Earthquake and you'll see why you are just overdoing it.
The Blood Magic spell Blood Wound, which boils the blood of all nearby enemies, as well as paralyzing them for the duration of the spell. Yup, another room-clearer, and one that makes all other damage spells parlor tricks in comparison. Some strategy guides actually advise against spells like this, since they turn pretty much every battle into walks in the park. The only limit is that enemies without blood are not affected, but there aren't enough of those to push it out of gamebreaker status. Also? This spell has no friendly fire.
Combine Blood Wound with Storm of the Century, and you're pretty much done.
There's some even more devious spells that the game knows go together in a Game Breaking fashion, and even mention it in the spell descriptions. There's Sleep, Horror and Waking Nightmare, Horror causing damage to enemies if they fail a resistance check, and Waking Nightmare causing, among other things, enemies to stop attacking you at least (whether through a fear effect or a temporary alignment change), and turning on each other at worst, also requiring a resistance check. However, if you successfully cast the Sleep on them first, they automatically fail the resistance check. At this point, your choices are, kill them immediately, or sit back and watch them turn on each other. Pass the Popcorn, please.
Force Field. Basically you get a tank character (some warrior, Alistair or Shale usually), get them to spam aggro generating abilities (as well as use aggro "aura" effects such as Threaten and Stoneheart) and once they have the unbreakable attention of every enemy on the screen, put Force Field on them to make them invincible. So, while 10 skeletons beat on your invincible tank character, your party is free to proceed as they please with no resistance to speak of.
Death Hex and Death Cloud, when combined, change from "Critical Hits For Others" or "Damage Over Time" to "DieBitch".
Sword and shield is also extremely powerful when stacked with dexterity, dodge + % and defense+ items. Making the tank nearly impossible to hit, add spell resistance from templar, enchanments, auto-health regen as well as awakening's broken carapace skill (First 15 sec of skill makes you immune to damage, the second half absorbs a percentage of damage that actually manages to land). For even more fun, give the tank a good dagger to take advantage of that high dexterity.
A more general Game Breaker is the Traps Are A Girl's Best Friend glitch, which lets you sell sets of spring traps (which require one extremely cheap item that is sold in infinite amounts at a nearby merchant) for a huge markup and gives you a hundred XP per sale. If you have the patience, you can easily fill out all your skills and abilities, as well as amping up your stats to ridiculous levels. Granted, the enemies level up with you, but the skills you gain more than make up the difference.
In a similar vein, once you have acquired the Dalish Elves' assistance, they ask for you to donate basic crafting ingredients. Like all donations to your forces, you get 10 xp for every item you donate. One of the items you can donate is elfroot, which you should already have plenty of and can get lots of for cheap at many merchannts...but what makes this not only useful but ridiculous is the fact that there is a single merchant in the base game that can sell you unlimited amounts of elfroot, in stacks of 99 (990 xp), for a pittance. It's Valathorn, the merchant...in the Dalish Elf camp. "You do not talk about Grey Warden Camp", anyone?
"We were giving the treehumpers' own stinking weeds back to them, and for it they made us gods."
In Awakening, Accuracy coupled with Aim, high dexterity and some rapid aim gear turns archery from the inferior rogue build (and rogues aren't that great to begin with) into having a crit rate of over 100% and tripling the damage you did before. What does this mean? You're oneshotting mooks at max attack speed with autoattack and are essentially untouchable. This allows you to even solo the hardest encounter in the game (The Harvester on Nightmare) with minimal difficulty. Which is good considering your allies for that mission are terrible and built wrong.
From the Feastday Gifts DLC we have the Qunari Prayers for the Dead: an item that can be used over and over so long as Sten in your party and conscious. It's effect? Bringing any fallen party members back to life. Very handy in some of the tougher boss fights. The real game breaking element is that you're invincible during the cutscene, and if you watch it all, the party is restored to max health and mana, so as long as Sten is alive, your party is essentially immortal. This can be cheesed to astonishing effect.
The ingredients for making Potent Lyrium Potions are much cheaper than what you can sell them for, making this an extremely easy way to make money. Even better is that you can buy infinite amounts of three of the four ingredients from a single vendor.
It's quite easy, with the right build, for a Rogue to obtain a 95% chance to dodge. This is just as overpowered as it sounds, and makes most non-Mage enemies a joke.
Essentially, the best party in Origins was three Arcane Warriors and a Rogue. Why? Because three mages is the most you can get and Rogues have a couple minor non-combat abilities that Mages do not have spells to emulate. Otherwise, Arcane Warriors do more damage than Rogues (see the first seven or so points) and take less damage than Warriors.