ok, jenova and bizarro were simple, just use alot of hit everything magic and materia, the best thing i found was get beta from the snake thing (enemy skill) and use that alot, it gets rid of evertything quickly except the right arm(bizarro), it absorbs it, but i was able to beat the first two forms in level 53 (or something like that) and with limited materia. dunno what is going wrong with your party, i sugest useing alot of All materia
with your other materia (fire, ice, and lighning work for this) another good one to use alot is comet 2, none of the bosses absorb it and it hits multiple times, another problem might be caused by the core, get rid of the core as fast as possible, because it stops him from using bizzaro energy, i still havent been able to beat his form afterwards, i cannot seem to figure it out, i am completely perplexed.