Secret Boss Help/Tips (PSP/GBA)

Well, I played this for about an hour last night after a looooong period of just procrastination, and I'm at level 54 now, getting one level every 15 minutes or so. And when/if I beat Chaos, I'm gonna take down the secret bosses I've failed to beat so far. Right now, the only ones I've beaten (out of accident) are Echidna and Gilgamesh, and they were freaking tough. So if anyone has any tips or words of advice for this multitude of tanks, it'd be greatly appreciated. And if anyone else needs help, some guides for Gilgamesh and Echidna would be awesome.

beat Gilgamesh, and fought atomos, but he was too tough. I did beat Chaos yesterday at lvl 45. Really long fight, like 30 min or so bc he used curaja twice, which is just cheap but whatever