For a mage, the best way is to use the blue wand, stand where the trees are in single file in front of you, and shoot so that all the trees are hit at once with a single shot.
By the way, there were several occasions where I found myself surrounded by a bunch of enemies, usually Ettins and Centaurs, and I had this urge to use the Tome of Power on the Arc of Death and just wipe them all out at once (enough to hit the Backspace a couple of times)...only to realize that I was playing the wrong game, heheh.