chaos: First thing's first. Don't use Ziggy unless you get his Executioner move. He is pretty much useless before that. Although, his amount of HP is better. Replace him with chaos or Rubedo. If you don't have them already, get the Double Buster and the BraveSoul. Double Buster can be attained in the KOSMOS simulator in the city area (With Shion and chaos). BraveSoul is VERY helpful when you are low on life. It does more than triple your attack power. Secondly, make sure not to use those "Attack Ethers". I was expecting so much when I had attained KOSMOS's Satellite, but it in fact does a minor amount of damage. Use the X-Buster instead.

Rubedo: Also, do what Hsu said. In the U-TIC battleship, the Ace Pilot gives you a lot of EXP points, and he is not that powerful. Other than that, train in the KOSMOS simulator, or try to avoid the enemies in the Song of Nephilim. And be prepared to fight some pretty mean bosses.