even if the first part sounds unbeatable, dont try it alone..

ok first you have to have cat nip (from the lvl 40 boss, the black element)

got that? good.

Now put it on a master gunner, along with the speed bracer for auto haste.

whack the gunner (probably yuna) with a sword or equip and de-equip 2 bloodlusts to make her critical

now use trigger happy and mash the r1 button like crazy for 9999 everytime.

this is NOT however enough

after beating that big omega weapon type thing, and having to fight trema, you should switch to using 5 mana springs (or 1 soul spring if you have it) to drain his mp. He can still use his deadly meteor but he cant use the even more deadly flare or ultima

Switch the other 2 characters to dark knights with crystal bangle, and crystal gloves, and have them use X-potions for healing (since you wanna keep yuna or whoever is the gunner, in critical)

it takes alot of luck even with this so keep trying. trema is VERY fast

EDIT: oh and its a big help if you put the timer and everything on slow, and wait etc. Im not sure but I think it even gives you more time on trigger happy