So it turns out I need a mind bogglingly high number of black crystals for all the new 6 star black magics. Like 586 black crystals.
Get 4 last standers and a radiant shielder (any non shield damage they do is going to be pitiful except for like piercing skills so it doesn't really matter who you take). Take dark resist and health boons so you aren't getting 1 shotted by drain - it's fine to recast last stand for multiple people but it's a waste if only one person needs it, especially if everyone else is at high hp. Bring hastega because you do want haste and if you mess up the orbs it'll go away. Feel free to let yourself get big banged in phase 2 by taking the timer to 0 or 60, but I found having some time leftover in phase 3 was good because zeromus will aoe you down with multi hit attacks and that feeds you more meter. Your win condition in phase 3 though is taking the timer to 0 and letting him obliterate your HP every few seconds while you refill it with shield up.
Curajas, curagas, memento of prayer and bursts are good to keep meter up for phase 2 but they're going to be osmosed away in phase 3, so go hog wild in phase 2 with your hones (except maybe save a hastega charge for dispel in phase 3).
Aside from that just get punched in the face and heal up and you eventually win. Zeromus's AI is predictable until about turn 58 (check reddit for the AI) and then it just goes weirdly random for some reason. Both phase 2 and phase 3.
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