Haste is a stupid good status and I'm one of those people that spends 20+ minutes in the soldier office at the start of the game getting my desired Zodiac signs and Br/Fa values for my party so I don't have issues with support spells missing. Demi 2 is hilarious against bosses, and Slow is a status that bosses don't resist automatically so it's certainly useful. Much more accurate than Breaks as well.
I mean it's easy to rule out things because FFT is easy enough where you don't really need to use many tricks to get by. If you're going for pure efficiency you're either running Math Skill or Summon Magic, but that gets horribly boring. I think Time Magic is fun to use and, if you really want to mess around, Yin-Yang as well.
I've played LFT more than regular FFT recently so I might be letting the tweaks mess with my judgment.
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No amount of planning will ever replace dumb luck.