do NOT use kotr on jenova synthesis and bizarro sephiroth. if you do, it boosts up safer sephiroth's hp by a lot.

first, have the destruct materia. you can despell or debarrier wall on safer sephiroth, because that's usually the first thing he casts on himself.

make sure a lot of your characters have hp plus', ultima might help but i never use it.

i love the enemy skill materia, mainly because of magic hammer and BIG GUARD. my main defense set up is always regen+big guard. it helps a LOT. but ur going to have to cast big guard plenty of times because it runs out fast.

if you even have kotr, go ahead and use it, but only AFTER you despell or debarrier the wall on sephy.