If you're looking to get around Cursed Body, there are 2 options you can try:
Go to the Move Relearner in the PWT and teach Leavanny Entrainment. This move will change Jellicent's Cursed Body to whatever Leavanny's ability is.
Another option is to get Serperior up to level 56 so it learns Gastro Acid, which will suppress the effects of Cursed Body.
The former is less time consuming, but giving Serperior extra levels may make Jellicent easier to take down once Gastro Acid is active.
Hope this helps!