What's a good build if I just want to blow everything up?

My main's a melee crit Static Strike Shadow, I've just started a tanky Ice Crash Mace duelist, and I figure I'll round things out with a caster.

I want to be a WMD Witch, glass cannon, major emphasis on nuking - cast my spells and watch hordes of mobs just explode. I'm guessing a Fire spell-crit build would be best for this task, but what should my main skill focus be?

crit flameblast

If you really want to play a glasscannon that goes BOOM, play discharge.

mjölner discharge
insanly stupid
insanly expensive
insanly funny
bonus:
looks like a disco ball and wreks your computer/fps ^^