F.O.E.'s are not entirely immune to the instant-kill aspect of skills like Head Pierce. Taking one down in one shot is very satisfying.
On a more general level, killing your first F.O.E. in any of the games.
Ronin in Untold have Severing Slash, a passive that may attempt to instakill all enemies at the start of battle. It can trigger against F.O.E.s. It's possible to kill them this way.
Interestingly, at least one of the franchise's vaunted Bonus Bosses, (the Fallen One in Legends of the Titan), isn't immune to One-Hit Kill attacks. It's just as amazing to see the colossal beast just keel and die from a Nightseeker's Assassinate, showing even the mightiest can be brought to heel.
Curse is by far one of the worst ailments in the game due to Health/Damage Asymmetry greatly limiting the amount of damage it does. That doesn't stop several players from making the most of it. Cue the Curse Cheese strategy, focused on inflicting Curse to a boss, giving a level 1 party member (with the weakest defenses) a Last Chance Hit Point effect, and letting them take its strongest attack. This results in Curse reflection damage that can one-shot a postgame boss!
Defeating the ultimate Bonus Boss at the game's hardest difficulty is another for the player, especially if they don't cheese it with a massive burst damage strategy.
Most games will, on victory, have the game congratulate the player for beating its greatest challenge. In Beyond the Myth, the player can opt to fight the Bonus Boss while Arken is around. Their victory in this case will earn Arken's undying gratitude, and the congratulations carries a little more weight coming from an NPC rather than the game itself.
Beating the Yggdrasil Core in The Millennium Girl, especially after the Post-Final Boss that is the regular battle at the end of Story Mode. This time, there is no Yggdra Vaccine. There is no Heroic Sacrifice to help the heroes. You're alone against an abomination M.I.K.E. believed required a multi-megaton weapon to destroy. You've weathered multiple attacks that would have caused a Total Party Kill. And you reminded it what dirt tastes like.
The Abyssal God of The Drowned City might give a little trouble at first glance, but once you've figured out its elemental rotation, it doesn't take too much to bring it down. But once you empty its health bar, the game displays "The boundary to another dimension is coming undone..." It then pans upwards to the Abyssal God's true body, and "Calling that Detestable Name" begins playing as you realize the boss is just getting warmed up.
The DLC Ur-Devil is incredibly difficult, more so than the Ur-Child, and capable of causing a Total Party Kill on a whim. Defeating it on Expert difficulty, where it holds back none of its unfair gimmicks, and while working through its attack pattern instead of just killing it in one go with a massive damage combo, is an incredible feat. The game acknowledges your victory with the Ragnarok sword, which at this point has become a Bragging Rights Reward.
Defeating the True Final Boss of Nexus gives the player one last cutscene showing its now lifeless corpse becoming one with the Yggdrasil. It's a sight to behold as the player's long journey through the game finally comes to a close.
It's not unusual to see someone try a daring Self-Imposed Challenge, usually a Solo-Character Run in a game that expects you to have a full party for expeditions, or attempting to beat the ultimate Bonus Boss with a single character or unusual team composition. The things those players accomplish can be a sight to behold.
In The Fafnir Knight:
The battle with the Demi-Fafnir in Story Mode. You begin the battle with the protagonist in his newly acquired Black Guardian transformation.
The final battle of Story Mode. The party has just beaten the Yggdrasil Core, but it reawakens and blows the party away. The protagonist has one last resort — the Holy Grail — and uses it to gain immense power, enough to curb stomp the Core alone.
The conclusion to the "Law and order" quest in has your party confront the thief who used the monster-summoning flutes against you. With his latest wave of monsters just beaten, he flees to an upper floor, only to be ambushed by one of the bigger monsters he had summoned with said flute. In the original, you then leave him to his fate and turn in the completed quest. However, here you have an option to rescue him and confront an optional boss. Doing the latter and beating said quest boss lets you turn the thief in to the authorities, letting you cash in on his bounty — 30,000 ental — on top of the other quest rewards. Satisfying.
"The golden shadow" quest suddenly places the Storm Emperor before the party. Bear in mind that this is meant for a level 60-70 party to take on, when the quest is accepted at level 45 or so. Fortunately, it does not engage the party, giving the chance to retreat to safety. Or, you could take it on... and defeat it, if you're cunning with skill usage. Bear in mind that you also need to keep the boss from escaping as it will flee after a few turns. If accomplishing that crazy feat is not already a reward in itself, choosing to fight the Emperor and win, despite its attempts to escape, will give the party an additional reward of 100,000 ental!
Periodically through The Drowned City you receive Sea Quests that send you to aid members from other guilds as they take on various bosses. Come the final Sea Quest, and you're tasked with defeating the Elder Dragon, and several familiar names decide to return the favor. They're also incredibly high level so as to not be a liability during the fight.
In Beyond the Myth:
All the townsfolk who assist you as guest party members in various quests are pretty awesome. Jenetta turns out to be pretty handy as a Rover especially in her second trip with your crew, Lili's Soul Evoker build gives decent support and helps prop up any of your own Necromancers with some of her skills and passives, Egar seriously WRECKS the Primordiphant with his Cannon Bearer build with a hefty BUSTER CANNON, Solor's Deathguard setup lets her layer debuffs for your team and her other skills synergize incredibly well as a damage support, and Merina's Spirit Broker build gives you additional firepower from the back row.
Special note on Jenetta's second guest star quest — When she rejoins you, she's now a powered-up Hound Rover with lots of handy arrow skills and some useful Hound healing, and she can easily pull her weight supporting your party during the ensuing battle(s), even against the Dryad.
The set-piece leading up to the fight with the Crystal Dragon on the 4th Stratum is pretty cool. You are brought before it in a large chamber, where it demonstrates the power to throw teleportation crystals at you as an overworld attack. You then have to approach it without getting battered by the shockwaves of falling crystals, ultimately using those same crystals against it by teleporting around it so you can close the gap before it can turn to face you.
Following that battle, the lead-up to the Fifth Stratum, and reveal that comes with it, is really awe-inspiring. Stepping past the defeated Crystal Dragon, Arken congratulates you and invites you to open the door to what comes next. Cue the cutscene that reveals the inside of a space elevator and the approach to the biodome that is the Fifth Stratum, complete with a good glimpse of the stars above.