Beat Lavos at the earliest possibility or if you want a tougher fight when he is Overpowered in Ocean palace.
I highly recommend that you do this on new game+, I would deem it impossible on the first play through.
Just take the right portal instead of the left in the beginning and you will be teleported to the fight with lavos.
Have the best gear you got from your last playthrough equipt, a bunch of high ether potions and the reduce mana consumption gear on chrono.
Chrono should mostly spam his illumate magic while marle start by casting haste and then use the barrier items. after that she make sure both are healed and has MP high. if she end up having no need for a support roll, she can throw in a Ice II once in a while and if both are on low health use Chrono as healer too.