While shimmer is somewhat op the real problem is that fire mages can get away with endlessly attempting to cast poly / pop all their burst in a tiny window upon landing a poly and win the match while being dead last in overall damage done.
Tone down fire damage or remove blaster master, a trait which hugely rewards using instant casts over casted spells and now the mage has to land 1 or even 2 follow up polymorph’s in order to score a kill which would increase the skill level required to succeed by a mile.