hmm...I think your talking about the end of the game, and once your inside his shell. I guess it really depends on your levels and your party. I never really had a big secret to lavos, but usually go chrono,Marle, Magus/Frog. Use Marle for healing and use your strongest techs. I think you kill the left bit first though to make it easier. The left bit can either heal or revive the other parts (i forget which one). It'll come back on its own but keep it dead