The Lich is the worst. I was in the 90s when I did these dungeons and I still had problems with the Lich. Everytime he kills you he heals like 30,000 HP. I wasted so many items keeping my party alive and the fight took so long. I did not have Gravity, which might have made the fight easier, but I eventually discovered something even better.
Throw an elixer on the Lich. This is final fantasy, undead get hurt by healing. I sure wish I had figured that out sooner. You are welcome.