So the attack pattern goes like this:

Start out with wisdom for the first couple turns. Anytime you see him self inflict 1 HP, you know that's the turn to switch to prayer. Cast a spell to force him to counter, heal up for the following black hole (if boosted, should be enough to cover it otherwise use healing magic)

After black hole you're free to heal or return to Wisdom, depending on where the EX gauge is. If it's high, I'll switch to Wisdom then Prayer. If it's low I'll stick with prayer.

Three or four cycles in, he'll throw a poison but it doesn't change much about the attack pattern. Plenty of time to heal.

The tricky part is when he gets around 25% HP, he will self heal. First turn is nothing, but the following turn at red counter 1 is going to be a nuke that must be defended against, no exceptions. Forget which turn are the nukes and you will die.

Hardest thing about this nuke is it adds two turns of HP drain, so you need to be fast about healing and it's a struggle to keep HP up. Skip the premium skills during drain--it eats up too much valuable time. You need to be able to reserve over 1200 HP for magic-flare-counter, then be able to heal enough to have over 2400 for black hole. If you can't consistently heal to over 2400 in a single turn then there's no hope of beating Zeromus.

When his HP gets low again, he'll start throwing meteors. This is the point where I throw a desperation attack in Power mode with Bartz' premium skill and boosted attacks.