Are you at level 30 or above? I usually beat Tiamat at level 20, so I have about 10 hours of leveling up to do before I'm ready for Chaos. I also don't "cheat" by cloning Excaliburs, so I found Zeromus to take about as many tries as any other boss in any other RPG. Some good bosses are: Dracula from Castlevania, Mr. Dream from PunchOut!!, and the Bydo from R-Type 1.