Lavos has a total of 12 forms, though most of them are trivial and can be skipped. Under normal circumstances, Lavos will take the forms of nine previous Bosses, right down to their stats. Because he uses their stats, these fights are extremely easy. In order, Lavos fights as:
You can heal up or change your party between each fight. After Giga Gaia, you will fight Lavos's actual form. (Depending on how you start the fight, you may begin at this form.) Lavos dishes out plenty of damage in this form, but doesn't have especially great defense.
At this point, you can enter Lavos's body. Inside, you can save or even leave via the Gate. Best of all, if you leave and come back, you won't have to fight Lavos's outer form again.
Inner Lavos has two forms. The first form has two arms. The Right Arm is weaker than the Left Arm. Status protection is important in this fight. There's no saving afterward, so use MegaElixirs at will.
Lavos's final form is just plain weird. It can be a tough fight if you don't know what to do. To win, you actually have to kill the Lavos Core on the right. However, it has powerful defenses that only drop when one of the other parts is destroyed. The Lavos Bit on the left has very low HP but absorbs all Elements, so hit it first with Physical attacks. After that, work on the Center Bit until the Lavos Core's defenses drop. When it restores any dead parts, destroy them again. Powerful spells like ✩Flare, ✩DarkMatter, and ✩Luminaire are very effective here.