The way I see it~ (sorted by the emoticons in the reply interface)
-> Discord + Black Hole. Classic, but can cause headaches, rainbow or not.
-> Eh~ can't think of any without resorting to Momentum the Death kiddies ... which makes the Vulture, given some access to death effect, a major threat (imagine the last time you've got a 15+/15+ mofo unstoppable by shields in either side of the field).
-> We aren't discussing mono Gravity decks. Move along.
-> Another classics~ Oty + Plate Armor, Pulverizer, Catapult + Basilisk Blood or Armagio, those kinda things.
-> Adrenaline + Momentum is going to rack some fast damage unhindered by shields. You can even fuel Momentum just by the mark. CC would be a pain, though~ T_T
-> Fire Eaters. Also, have you tried to handle a 13/4 Crimson Dragon shields can't block? I have yet to.
You damn hypocrite~ -> Casting Momentum on the Water creatures don't seem like much, but that lets those watery beasts get past shields. Water-types has always been notorious for its PC (or, rather, lack thereof). Also, Purify + Overdrive = highly problematic creature that grows +2/0(+1) indefinitely without hindrance (read: CC, either in the form of RT, lobotomy, or outright killing the thing).
-> I don't really think Guardian Angels' creature heal can get you pretty far with Armagios (granting him maybe one extra turn of absorbing attacks), but Overdrive is another story. (Sapphire Chargers are fairly difficult to get around defensively. Now imagine one that grows practically indefinitely.)
-> Without resorting to Momentum or Flying Titans (Flying, not Animated, if you know what I mean), I can't really think any.
-> SCARABS. Nuff said. If you have no Pharaoh yet, see if RT-ing a Mummy (is splashing Death safe for this?) can serve you better.
-> Did I heard 'quanta denial'? BH + Devourer can deny quite some resource. And if they thought his Sanctuary can save his hide from the Grand Quanta Theft, well ... see if they can still deny quanta denial after you stole his Sanctuary. XD
-> Momentum + Quintessence = completely untouchable PITA. Since Momentum is pretty cheap, you can even splash it while you're using the two of them to something even more devious. (*wink* Crimson Dragon *wink*)