Just make Archmage boost damage and costs only

Please stop trying to make it scale with mana. It is never going to work ever. It is way too volatile and you can't keep up if you do anything else because there are so many ways to get shitloads of mana.

Just make archmage increase the mana cost of spells by X% and then you gain Y% extra on top based on gem level, and leave it at that. That way you can scale your damage output with mana regen and pool by simply being able to maintain the casts.