It is very possible without holy war/trial, hero/trial and invincible moon.
I've tried it, it is easy
Rinoa have to be in 255 DEF (so she could survive the attacks and revive the others)
Squall and Zell with 255 ATK
Other STATS have to be above 130! (keep them as high as possible, using magic junction, devour (Eden's ability), or UP item such as Vit-up, Luck-Up, etc (get them from refine)
you may want magic, revive, recover, auto haste, auto protect, auto shell, etc on your ability.
the party must have item and a bunch of megalixir!
Battle:
First use meltdown/ doomtrain. Then, Cerberus, if you'd like.
Just keep everybody's hp 9999 everytime, and
use aura on Squall and Zell often.