Hmm I must be doing something wrong then - in my current Wizard playthrough, my PC and Gale are usually the last two standing (I decided for a different route than usually: went straight to the Risen road - took on the toolhouse gang very early on and was actually kinda surprised it worked out).
I second the others - mage armour, Misty step and Mirror image are your best buddies - even before ofenssive abilities (I actually LOVE Astarion, Gale, Wyll and my Wizard setup)
