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Goblin Punch costs 0MP, and usually deals average damage, but deals many times that if the caster and enemy are of equal level. Other times those one mooks often go down with this spell from the start. In earlier games, it was very easy to manipulate the enemy's level, and in every game that let you do this it also bypassed normal death immunity. L5 Death is guaranteed to work on enemies with a level that is a multiple of 5, including normally immune enemies and bosses in some of the games. This can be something from the difference between current and max HP or just an insanely powerful attack. A spell that does more damage if the user has less HP. Bad Breath will attempt to inflict every negative status ailment in the game. White Wind always heals at the current or half max HP of the user, bypass Shell and Reflect checks, and may heal status effects. Normally the actual magic that applies these buffs can only single target and cost more MP to cast total than Mighty Guard.
Mighty Guard at the minimum gives the team physical and magic defense buffs.
This is useful if you either come across this early on (and survive getting hit) or if you face an enemy that has absurd defenses, but low HP.
1000 Needles does exactly 1000 HP of damage, no matter what. A lot of these however, do require some Guide Dang It!. Enemy skills (or Blue Magic) from any given game tend to be abused due to their potent effects at the earliest they can be obtained. And completely ineffectual if you play in Wait, or even Recommended. In VII, effects that drain HP over time (like the Waterpolo's attacks) and things like Death Sentence are actually quite rapid and threatening on Active. This was specifically nerfed in the DS remake of Final Fantasy IV, wherein the ATB pauses every time a player character performs their turn's action. Not to mention actions that ignore the action queue and take place immediately, such as Quick turns in V and VI and Limit Breaks in VII and VIII. Or you can simply pause the ATB while your characters are performing their attack animations, while leaving it running while the enemy performs theirs. This is a great way to curve when the enemy gets its turn.
If you put it on Wait, input a few actions but keep one in some menu (items, magic, etc.), the action queue will empty out and then the next character will have their turn immediately.
The ATB system in general can be a game breaker if you know how to abuse it.