How did you manage the fire giants? At 16 mana each were you able to produce them all with your sovereign (before you took hs essence that low), 1 per champion? I had thought each champion would need to cast his own, and you wouldn't have sufficient mana
I have followed a similar if less extreme strategy, took 15 each int/wis/char on my sovereign, imbue all of the champions I find without letting sovereign essence drop too low. I take then to 10 essence (enough for 2 chain lightnings or 3 single-target spells) and then up their int.
The total mana regen is so much better across multiple casters that it can carry you through a long way into the game
