It'll actually go by extremely quick. You can make a circuit of the Hellmouth, the little base area off to the side of it, back around the mushroom building, and through the starting area to go back to the Hellmouth and probably have 128 in an hour and a half. Just check every cave and weird places for chests, and make sure to keep your ears open for the sparkly sound when you're running around. I usually pick up the body count and item collection bounties as I'm rolling through farming, and you'll get some XP/rep while you do it too.