k so. i had this problem also.
use , yuna tidus an lulu.
tidus cast hastega, lulu cast firaga everysingle time(unless u get overdrive) yuna just to heal every1 and aoens when ur party got full life.
first form- ez pz, just attack normally, however, when lesca has low life make sure ALL ur chars have full life/no status effects on them.
second form- keep 2 of ur chars in zombie form, dont use a holy on them cos hellbiter will be used again. if 1 char dies then revive and use curaga as soon as possible before hellbiter. this is real important:
WHEN UR BOUT 2 KILL LESCA MAKE SURE AT LEAST 2 OF UR CHARS ARE IN ZOMBIE MODE AS HER MEGA DEATH SPELL KILLS EVERY1 THAT IS NOT A ZOMBIE.
3rd form- all aoen overdrivess here, quite ez rele, same tactics as 2, keep 2 ppl zombies so mega death wont get u game over. her attacks dont rele hurt at all, low dmg, so just keep attacking as normal. also keep an eye on lulus mp, i had to refill it twice so just watch that.
hope this helps