I think you should switch the costs. 12 for a near indestructible creature (I'm assuming it nullifies damage?) that can't safely retain one skill, while 4 (maybe a bit higher?) for something quite killable.
Interchangeling can actually quite easy be, let's say, "lobotomized", while you can't do anything to protect it. Annexor for instance could be targeted with something, then fractal'd. Next turn you blow up an Unstable Gas and all your Annexors can immediately sacrifice themselves for 20 damage each (assuming your opponent does not have a reflective shield). To prevent this abuse, or the fractal + quint + Annexor combo so you can fractal each turn, i made him cost 12
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This is beyond OP, and you know why?
Bolt spells
Blessing
Lightning
This is why i put Annexor's cost so high, should it be higher, though?