You could cast greater mighty wallop (Races of the Dragon, p115) on blunt arrows (Dragon 249, p23 or Races of the Wild, p165). That ultimately ups the damage to colossal. With the chain spell feat (Complete Arcane, p76) or a lesser metamagic rod of chaining(Magic Item Compendium, p165), you could get a bunch per casting. With a greatbow (Complete Warrior, p154), that takes you from 1d10 damage to 6d8 damage. You can take a -4 penalty on the attack roll to convert the damage back to lethal.