I think picking a spawner that is closer to you is a very reliable source of XP. I made myself drop farms from those, so the mobs can get some damage, making them easier to kill. The idea is simple - just make a 3x3 or 5x5 (bigger = better) hole under the spawner, with a platform for the mobs to fall on, and just enough room for you to be able to hit them.
You have to be in range of the spawner, within 16 blocks, so don't go too deep when you dig that hole - just enough to maximize the damage the mobs get. And to make sure they don't spawn outside the drop zone, close the dungeon and light up the places outside close enough for them to spawn.
simple but effective :)
P.S. This will change in the future, but currently the mobs drop XP when they die. So if you place magma cubes where the mobs fall, they will die from damage, drop XP orbs, so you can just idle with your tool that needs mending in your hand, and come back later to switch to another one needing mending.