I just did tests with that yesterday. Splash Healing potions thrown from a RP Deployer will yield both skulls and XP. (I haven't tested vanilla dispenser, try it if you don't have RP.) You can make healing potions automatically. For glistering melons, use either a Zombie Pigman farm, bees, or just throw in gold manually. Pipe the potions to a dispenser, and activate it on a timer. Drop the skeletons from high enough that they're one-hit kills. As was already said, collect drops with Transposers or Obsidian Pipes, and XP with a magical device.