I just defeated her with a lvl 52ish party. I don't remember what level that's normal here but 52 sounds about right, no?
SPOILER alert, in case you want to solve this yourself.
My team: Coud, Vincent + Tifa. Why Vincent? You could replace him with a character with more HP, but his Natural Spr helps a lot.
Cloud front row, cause he had no long range. Vincent back, Tifa front.
Eq:
Cloud: Nail Bat, Gigas Armlet, Fury Ring
Vincent: Lariat, Wizard Bracelet, Peace Ring
Tifa: Dragon Claw, Bolt Armlet, Peace Ring
Materia:
Cloud: Anything goes that raises luck, speed and HP. NO magic or any materia that lowers Str.
Vincent:
Weapon: All+Restore, Magic Plus, Revive, Enemy Skill, MP Pluss (Magic Plus and MP Pluss aren't really needed though, you may want to replace MP Plus with Counter Attack for extra damage utput, IF you have one).
Defence: Elemental+Fire,All+Time,Added Effect+Destruct,HP Absorb+Comet.
(He has less than 3000 hp now).
Tifa:
Weapon: All+Barrier,Counter Attack,Long Range,HP Pluss,Enemy Skill
Defence: All+Restore,Elemental+Fire
Started the fight with Sadness on all with tranquillizers.
At the beginning of the fight I prioritized MBarrier and regen. Dragon Force twice on everyone to lower damage (of course, watch you hp, you don't' want anyone to die).
Then add Death Force on Cloud and Tifa as fast as possible. Make sure they don't miss.
The rest of the fight is simply a matter of keeping your HP, regen and mbarrier up. Both Tifa and Vincent can do this; no worries.
If you're unlucky and Jenny keep spamming successful death spells at the beginning, you might as well give up. It have happened to me too.
EDIT: btw, you can also cast Slow on Jenova.