That One Attack
A particular attack in an enemy's arsenal which is significantly more dangerous than most of its attacks.
If you have a party, it will almost always target all party members, and it often takes off the majority of your hit points, if not worse. It's a staple for That One Boss to have one of these, possibly more.
This attack showing up even once is always bad news; if the AI Roulette gives you two in a row, expect to see the Game Over screen. Whether or not you can beat That One Boss that uses this attack usually depends entirely on whether or not you can avoid/endure/recover from it.
Skill Gate Characters may also derive their harmful-to-newbies nature from having one of these, and thus falter when the opponent learns to counter it. If these are available to players without suffering Redemption Demotion, they usually become Game Breakers.