Rainbow can be cleared by pretty much anybody as long as you have a healer set as leader and a (green) Odin friend. The first four stages are not particularly difficult and Rainbow Keeper cannot actually kill you if you have ~5000 health or more and can heal up to full every 4 turns.
It takes like an hour to clear it this way, but getting a Rainbow Keeper is worth it. Not only are they needed for several evolutions of powerful critters, they start out at max level and have 3126 HP and 1026 ATK right off the bat.
Edit: Most of the damage you'll be doing will be from Odin's skill, which is why it takes so long. If you happen to have a Lilith (or even better, a Neptune... or a Hades) you'll be that much better off. Otherwise, just bring your best stuff (preferably things that have skills that do damage/heal/reduce defense)
I learnt that you don't even have to move orbs in straight lines :V
Jesus christ pesco :V :V :V