Rainbow energy is treated as all energy types, so it still has to be discarded after using Flare Blitz.With regards to the Giratina issue, is there anything we can use to turn on Talonflame Break's attack? Then we just OHKO it.
On the low tech end...I don't know...Rainbow Energy?
Not saying it's a great solution, but it could work. Also gives us a Pokémon that retreats for free and hits for 150 and, if I understand correctly, since Rainbow Energy counts as colorless energy, it isn't discarded when you use the attack.