You can either face Lavos before Marle is teleported to 600 AD so you can fight him with both Crono and Marle, or you can fight him with Crono solo if you face him after Marle has been teleported away. He shouldn't be too hard, especially with Crono and Marle. Just make sure you have status protection, and Gold Studs would be a good idea too.

IIRC, Marle can speed things up with Haste. Have Crono use Luminaire and his tech that hits the enemy four times (was it Confuse?). Marle can heal, or you can have her and Crono use Aura Whirl, Cure Whirl or whatever it was called. She can also help with her Ice spells. I can't remember if there's an item that casts Protect, but if there is, use one on Marle at least during the last phase since Grand Stone, the ultimate physical attack, will hurt her big-time if she's not protected.