Yeah, I defeated the second form at level 52/53 and Vaan was 60. It was easy, just use your Gambits correctly:

One person= Oil>>Firaga on Enkidu
Reddas and another person= Slash at Gilgamesh
Ally 60%>>Curaja

Just do that until the dog's dead, and dispel everything the dog's got as well. All you can really do is just smack at Gilgamesh, and heal automatically (have a white mage, preferably a Gunner/long ranger)