Things that make Rainbow decks overpowered, solutions.

[JTWood]

you can save up Crimson Dragons and spam them like direct damage spells.

Until the never-goes-away-because-fire-doesn't-have-enough-critters bonewall goes up.

Then those Crimson Dragons are just omnom food.

[Parabol]

Fire played well destroys rainbow.  Rainbow tends to have one or two creatures at high health, which fire is designed to deal with, along with a few weaker creatures, and an attempt at generating hordes of weaker creatures (rain of fire).  It has no defence against spells, and the great length of time it takes to play means you can save up Crimson Dragons and spam them like direct damage spells.  His otyughs get +1/+1 per turn.  You kill him in his face.  IN HIS FACE.

Fire Buckler alone severely limits the decks.  I don't think PVP is unbalanced, and I greatly enjoy PVP.

Poison also gives Rainbow problems, for the same reasons scorpio does, because most of its damage completely ignores all the little shields put up.  Purify is a counter, sure, miracle is a counter, sure, but they're two cards in a deck generally between 50 and 60, or else a vast disadvantage in a smaller rainbow deck.  Haste is a small modifier to these odds, IF the player has included them.  I believe a player-created poison deck would be more problematic for rainbow than scorpio, as preventable damage such as arsenic and puffer fish would be dispensed with to focus on direct poison damage from chrysaora (unupgraded to prevent fire wall/procrastination/ice shield retribution) and deadly poison.  Arctic Squid would be nice, but congeal does the job.

I'm greatly impressed by how balanced the game is, with almost every kind of deck having an edge against some elements, and weakness to others.  Rainbow is an exciting kind of deck to play, because the combinations appear limitless and sundial is powerful.  But there are always decks that are designed for a more specific purpose, that will be equally powerful against the decks that they target, such as aether with its philosophy of inevitable damage, or dark/time denial decks, etc. etc.

Sorry for the essay, but balance is one of the things that most struck me about this game, and I just don't think that Rainbow is overpowered in PvP because it can stall, any more than Gravity is overpowered because it can break shields, or Light is overpowered because it can heal up to 218 health in a turn.

[Parabol]

The idea if otyugh OR bonewall come down is not to play your creatures, and play Firebolt instead.  Rainbow decks have VERY slow damage.  The most popular deck's main creatures are either growth, devour, or spawn.

FFQs can be annihilated.
Otyughs can be ignored by not putting creatures out - or only using immortals.  If they DO eat your crimson dragons (say, there's two otyughs and no defence out: you can spam 4 dragons at once), it's NOT an instant loss.  The damage dealt will still be at least 60.  The benefit to the Otyughs is a growth of +1/+1.
Growth creatures are weak to a rain of fire, that can be brought down with confidence because we KNOW that successful (i.e. haste-abusing ) rainbow decks tend to have few creatures.
Fire wall will eventually destroy all their creatures in any case - you could even feed a human opponent's lone oty a beast to get it to kill itself in this way.

This leaves lycanthropes as a possible entropy entry (Fallen Elves can be ignored as 3 dmg/turn),
and graboids.

I think graboids are the only realistic threat that rainbow has against a fire deck.  Certainly, it has weapons capable of taking on death - plague and retrovirus are just slower than rain of fire.  But the highest damaging creature a rainbow can release vs monofire will be a 5/4 immortal, which even then has a one-turn window in which it can be damaged, and likely will.

[Uzra]

If I were thinking of rainbow decks designed to beat gods I'd agree, they have tons of weaknesses.  But It's not hard to modify them for PvP and squeeze in a 2nd miracle and / or a few power artifact.

[RoKetha]

You forgot the part where they steal your shield, the part where you have 6 deflags tops for 3 phase shields and 6 sundials, the part where they get 2-3 cards per turn to your one, and the part where they heal a small amount every turn even if you do a pretty good job of controlling their creatures. They also get to completely control the pace of the game; creatures only attack when they're at an advantage for the most part, and when you get a creature advantage they have many ways to get around that, including Bone Wall which is almost certainly going to screw you up horribly, or their own rain of fire which WILL one-hit all your dragons.

I've tried both immolation and towers in a mono-fire deck and the game just looks like this: sundial, sundial, phase shield, phase shield, 0/11 oty, sundial, enchanted eagle's eye, bone wall, graveyard, empathic bond, FFQ, miracle, etc. The rainbow decks people play most are specifically designed to counter the creature blitz tactics that gods use, after all.

Rainbow also always just always ends up with a huge draw advantage over fire no matter what. It takes a lot of luck to pull off a win against it; your best friends are usually going to be momentum and immortality for attacking, not the fairly meaningless attack stat. Heck, gravity and aether dragons are way better than fire against rainbow and it doesn't take a lot of insight to see this.

Edit: Yeah, fire bolt does work, but it takes roughly ten turns of doing nothing to save enough quanta for even two bolts to actually kill them. Anything less gets Miracled away, and an eagle's eye by itself will knock out 70% of your health in that time... keep in mind you have no healing.