Zeromus

[Rengori]

At least this is the last time I'll need help!

[Momiji]

Uhh...why don't you ask about the problem in the first post than expecting help when we don't know what you're having a problem with?

[Rengori]

Defeating him dammit! Stealing the Dark Matter didn't seem to help much either.

[The Man]

Try leveling up by quite a bit first. You need to be at least level 50, and probably higher, to beat him. Also, use Rosa to cast Cure4 (or whatever the highest Cure spell is; I think it's that) every chance she gets.

[Momiji]

I leveled up Cecil to at least level 70. (This is the hardtype, by the way.) I leveled Rydia and Rosa until they had their final spells (Meteo and Holy), and I just didn't do much with Kain and Edge, Kain always jumped and Edge usually survived so I could steal D.Matter, throw the knife, Excalibur, and a couple fuma shurikens until he died, then I never revive him, he dies too fast.
Before you start the battle, have Rosa Protect/Shell everyone, and have Edge steal the D.Matter. Then use the Crystal.
Have Cecil cover Rosa, and have Rosa heal/revive every turn, and if you can, throw a Holy into the mix (it will do at least 4000 to Zeromus.) Have Kain jump all the time, and have Rydia use Flare/Bahamut. Just make sure you can take a hit from Zeromus's Big Bang at all times and you should be fine. (It's kind of funny when Zeromus loses all his MP, all he can really do after that is attack physically or use...I think it's called Black Hole, which is just a mass Dispel spell. He should go down easily after that.)

[Kawaii Ryûkishi]

Sheesh.