As s mage for bosses charging anything lowers dps, but as Aedenos was saying I charge my auto attack when dealing with 5+ mobs. Reason being is I usually have aggro and this gives me time to kite, couple w with the stun occasionally brought by charged AA and increased damage and larger AoE effected.

This strategy I normally use in hauntlet, watchers, tombs, and against piggies in towers. Spam attack suffices 90% of thettime.