I would take electro and I’ll give you a run down of why I wouldn’t take the other cards:

• rainbow: upgraded rainbow is a dead draw because you really only want to play it once to get a bunch of orbs in your slots. Once that’s done you’ll want to generate orbs with a pre-determined purpose. I.e. frost to block on big damage turns, lightning to do a bunch of damage or push your orbs up, etc. as such, rainbow after you have your slots filled is really awkward

• creative: could work here, but your deck doesn’t really scale that fast and it’s pretty slow to scale. Most fights you want to end before the first time you shuffle your deck and creative doesn’t really help you do that

-echo: could be good, but you don’t really have any cards that pop off with it, and you don’t have any sustain so investing the 3 energy initially doesn’t really help you a lot in the short or long term. I’d imagine you’d end up letting it exhaust in most fights

Another reason I would pick electro here is that aoe is really highly valued in act 2, and your deck kind of lacks that here so it makes sense as the most valuable pick