Use water streams to push mobs into a "bubble elevator" (soul sand was not an elevator block in 1.7.10; you must use alternating water and signs) to lift them up about 22 blocks.
Side View: [Spawner] Y=64 | | (air) | [Hole] Y=64 (1x1) | | 22 blocks of air | [Water stream] Y=42 (pushes skeletons to kill spot) | [Kill spot] Y=41 (player stands, hit skeletons) | [Hoppers + Chest] Y=40 minecraft 1.7.10 xp farm
Blazes drop 10 XP – the highest of any "common" mob in 1.7.10. However, they fly. Use water streams to push mobs into a
Find a Skeleton or Zombie spawner in a dungeon. Skeletons are preferred for their valuable bone and arrow drops. minecraft 1.7.10 xp farm