For maximum damage, I'd put two damage enchantments on it - fire and shock, or fire and absorb. (If this is possible. I haven't gotten to double-enchant anything yet.)
Or, join the Thieves Guild and get the Nightingale bow, by far the best unique bow in the game, though getting it at later levels is better than earlier. If you get it between levels 1-19, it'll have a base damage of 12, do 10 points in frost damage and 5 points in shock damage. But if you wait until level 46, it's base damage will be 19, it'll do 30 points of frost damage and 15 points of shock damage! And of course you can improve it at a smithy, with void salts. At level 46 youll probably be high enough in smithing to get it to legendary right away, and then it'll do a ridiculous amount of damage. With a couple of archery perks and the 3x sneak attack perk, maybe one piece of gear enchanted to improve archery, you'll be able to one-shot almost anything.
Oh, for a grand soul gem, buy one off a court wizard and go kill a mammoth. They have grand souls. Or, if you can't quite kill a mammoth yet, do The Black Star quest and get the Black Star, then kill any human. Voila, grand soul.