So what you do is build a huge enclosed tower
At the top, which should be exactly one block less than what mobs will die from falling to, use a redstone timer system to push spawned mobs down the big opening, which should be only two blocks wide. At the bottom, you want to have stairs going up to I believe one block and a slab beneath the opening, which should only be a slab high in order to prevent spiders and baby zombies from escaping. You want to make sure these mobs will almost die when they hit the ground, and after killing them, run around the entire thing to get the xp
