If you upgraded Fire Breath and like it, I can't talk with you. This is the worst skill of the whole game dude, seriously, 4 fireballs deal more damages than this.
And you know, When you are a Wiz, all you need is : Mana, fireball, Meteor, Ice shards, Shield (not armor) and HP (combo of course, that's for all the classes). With all the secrets and stuff you'll have some extra money to upgrade other things.
But seriously, Meteor is the best, it "just" one shot all the monsters you have on your screen. If that's not the best skill of the game... I don't know what it is.
And about mana consumption, it's really no big deal, because the wiz can do the whole game with his fireballs so... Just wait for the mana bar to replenish.