I killed Alduin in one shot one time. Daedric Bow enchanted to do 25 points of both fire and elecric damage, smithed up to legendary, using a Daedric Arrow, and using my 100 Alchemy (and heavily enchanted gear to add even more to my alchemy) to mix (the one and only)Jarrin Root with some Nightshade. The poison alone did ~6000 damage.