actually, i beat the jenova, bizzaro, and safer at level 50 (yes, even Cloud!). all i did was use Cloud, Yuffie, and either Tifa or vincent. Cloud was stuck on Climhazzard, Yuffie on Lanscaper, Tifa on Somersault, and Vincent on Death Gigas (poor, huh?)
Cloud had Ultima Weapon, Mystile, Ribbon, 2-cut, W-Item, mastered Restore, Ultima, mastered Lightning, Time+All, mastered Barrier+All, and Bahamut
Yuffie had Conformer, Minerva Band, Ribbon, Slash+All, Throw (in case you have extra weapons), mastered Ice+all, Destruct with DeBarrier, and Neo Bahamut
Tifa/Vincent had Premium Heart/DeathPenalty, Mystile, Ribbon, Contain with Flare learned, Heal with Esuna learned, mastered Fire+All, and Bahamut ZERO.

if this doesn't work, you need more training. i suggest that, at the fork in the northern crater, go left, then up at the next fork. you'll run into some good and easy enemies (movers and magic pots). movers won't give you exp, but you'll get 2400AP and 90,000 gil! as for magic pots, they wont attack as long as you give them an elixir. you'll get 8000exp and 1000 ap for each one you kill. if you run out of elixirs, you'll run into master tonberrys and steal an elixir from him, then run. Find this helpful?

Now, when fighting bizzaro, keep killing its head when its revived cuz it knows Heartless Angel. Then just hack away at him.
for Safer Sephiroth, cast Wall+All on yourself and use DeBarrier on him. keep your health at least above 2000 hp cuz he'll cast Super Nova when he floats up. Heal yourself then massacre him. You should be able to beat him.

Find this helpful? Hope so...if you fail, then i'm sorry, but this is when i was playing. Good Luck!