Ok if I were a wizard and was going to overcharge a spell, it would probably be cloud kill. It's an AOE that lasts 10mins. There's nothing in the description that it had to be a spell that only does damage once. You get creatures trapped in an area and they will take 40 or 20 every round until they die.
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I see this as a tremendous idea, especially if you have synergies with other characters forcing baddies to stay in the cloud. I always found Cloud Kill lacking, but with overchannel definitely not the case. Good find.