This spawner gets you tons of XP.
Step 1: Gather Materials
Get water, lava, flint and steel, mob spawning stone, and obsidian.
Step 2: Base
Make a 3 by 10 row of mob stone and hollow out the middle 1 block deep and fill with water, and at the last block deepen one more block and fill with lava. Then surround with obsidian.
Step 3: Portals
Make portals around the structure.
Step 4: Wall
Add 2 more obsidian layers around the structure and build up the last layer.
Step 5: Lava
Put lava in between portals and wall.
Step 6: Extra Layers
build portals on top of each other as many times as you want (more layers better effect).
Build a portal in the middle.
Step 8: Stone and Water
On top of the portals build a stone layer in between. Then build 2 high wall of obsidian. After you put water so it flows inward.