I may be reading this wrong, but if there is a finite amount of ambient mana in a given place at any time, wouldn't it be theoretically possible to temporarily drain a place of mana in advance, and lure your enemies to a battlefield where their mages are cut off? Alternatively, you could have your own mages drain the battlefield with useless spells, rendering the opponent's mages moot as well over the course of combat. I suppose the amount of mana is too large for that to be considered a viable strategy, but it's something to think about.