How do you beat zeromusEG?

I can't help with ZeromusEG since I won by luck. But, I can give some advice about defeating Lunar guys if you are meaning Lunar Odin, Lunar Titan, Lunar Asura (the one with three faces I'm not sure with the name). Defeating Lunar Odin is actually easy if you know that his earthquake doesn't work on land. So, all you need to do is have 2 white mages which both know fly as soon as the battle starts, use fly then just have fun attacking him without counter each time he puts you back to land, just use fly again. Soon, he will die and you'll get the LT to summon him.
Defeating Lunar Odin needs a little trick. Try to attack him in anyway to lower his health while also healing your party. When his hand raises, which means he is about to use zantetzuken, get edge to use blitz.After that, there will be a message and Lunar Odin dies. Then, you'll get the LO to summon him.
Defeating Lunar Asura also needs a little trick. The trick is using reflect. Use reflect on Lunar Asura at the start. Then, try to lower her HP without magic of course, as fast as possible before the reflect wears out. And when she heals herself, the curaga will instead heal one of your party. Then all you need to do is maintain the reflect while finishing her.Hope this helps!

I think with Zeromus EG, you have to be at an extremely l(maybe 60 or higher), but he has double the HP (200,000) and heightened attack power as compared to the original Zeromus. Don't use magic--he'll counter those. Any low HP characters, like the mages, will be slaughtered by his ability to use multiple strikes. When he gets down to roughly 20,000 HP, he'll use some regenerative ability to stay alive, but if you have Kain in your party and a lance that can put ANY emeny--including Zeromus EG--in HP critical status, attack with that and use the Jump ability to deal more damage than Zeromus EG can heal. Most of the battle is luck, but heal often and attack with physical attacks rather than magic.