In isolation, I think this could make for a neat troop.
Doppelganger
Mimic (7 Red/Purple): Assume the spell and Magic of a target enemy troop. That spell replaces my spell (colors and cost). After the spell is cast, revert to Mimic.
(Obviously this would need tuning for the spell cost.) This could theoretically give you a repertoire of 7 spells in a fight, at the overhead of priming Doppelganger each time. Note that Mimic would copy the Magic stat only; all other stats would remain unchanged. If that is too strong, perhaps Doppelganger would always have a fixed, low Magic score, which would limit his usefulness (some spells would be great to copy, like Sandstorm, while others would be mostly useless).