Make sure that you have all your characters keveled up enough. The higher, the better. Assuming you've done all the character side quests done (some of the items, weapons and armor can really make it easier) try and learn the triple techs. Lifeline basically makes it so each character can die and then get back up.



Also, on tbe final form, whenever the lavos core resurrects the other two, attack the left-most one with physical attacks. It only has about 2000 hp, and this will keep the core in (supposedly) perpetual resurrection state where it's defense is down. Other than all that and stocking up on all the Megalixers you can get your hands on, just hit him with your mightiest techs. Nothing else.



Also, mak sure you have panceas on hand. Those status effects can really suck...