Meteor kills your whole party? Even at LVL 60, that's pretty hard to be so. Even if it leaves you at very low HP, having two White Mages can easily restore you back to full HP.

And you need to keep restoring your Shell, that should be your top priority once Rosa is fully healed.

You're mostly having bad luck, but that doesn't make it impossible. I defeated him at LVL 65 or so and it only took me two tries (a bad timed Kain's Jump was the fault for not doing it on the first try). 

Meteor does 5000 damage without shell, and I'm pretty sure Meteor ignores Split Damage like Big Bang.

And everyone's Lv 60-65.

I don't remember it doing that much, mostly in the 2000-3000 area. Must have been because I always tried to keep Shell up. 

Oh, and have you been using Slow on him too? 

Edit - And I also forgot. Meteor only kicks when he's at 30K HP or so. At this time, just throw all the strongest items in the game with Edge and simply go all out on him.  

I can't keep slow on him!

He uses black hole and big bang! Rosa dies, can't resurrect her or I won't be able to damage Zeromus at all. And Black Hole wipes him clean every other turn so resurrecting Rosa so often means I can't damage him.

And yes, Meteor does that much.

Here's the strategy I used until Meteor fucked me up. Three times.

1) Wail on Zeromus, talking Fuma Shurikens, the best weapons, Bahamut, just SLAUGHTER him

2) Zeromus shakes

3) Kain Jump

4) Party Dies

5) Kain Lands

6) Kain uses Phoenix Down on Edge (he can deal the most damage the most quickly) unless Zeromus shakes

7) Elixir or Kain if hit by Flare

8) Rinse and Repeat

9) PRAY TO DEAR GOD THAT YOU CAN AVOID METEOR

10) Die

It works pretty well, until the whole death part.

This is pretty much the only consistent strategy that works to deal damage to him.

Dude. 

Phoenix Down on Edge? Why? 

Since Edge can hit for 8000 and there's literally no point in resurrecting anyone else since they can't hit as much and healing, buffing, or debuffing serves no purpose after ten seconds.

If you revive Rosa first, you can free Kain for the attacking role. It may not cause so much damage as Edge, but you get to keep a safeguard. Plus, I believe there's a Osmose step somewhere in the battle phase when he's only using Big Bang. That should be your opening for any buffing/debuffing. 

Also, when he starts spamming Meteor, he lays off quite a lot from Big Bang as well, and has a tendency to use Osmose more too. Burn through your Elixirs if you must. 

Do you not understand what I've been trying to tell you?

Meteor kills me outright and so does Big Bang!

He uses Meteor right after Big Bang, I think, but I don't know when I can't do one thing about it to even remotely prepare myself.

Black Hole = Defenseless Kain

Big Bang = No way to restore buffs

Meteor = LOL, I'm completely unfair!

That's why I'm saying that ressurecting Rosa is a top priority when you get to the Meteor stage. 

You need Shell, even if he constantly dispels it with Black Hole. And if you can keep Slow up then it's a plus.

He follows a strict battle pattern that you should have figured out now, if you have been dying that much. Make use of the gaps between attacks to prep your strategy. I just don't know what else to tell you, because I've already went with you all that I did on the battle. 

All that I've figured out is he uses Meteor when his HP drops enough.

But the thing is I have no CLUE when that is.