How is it done?
You go alone to Zul'Farrak in Tanaris and ride to the graves. Here only close combat mobs are spawned which are stabbed and then flattened with AoE.
It can be filled and reset to the boss (next to the boss jumps around the corner in the direction from which you came to the podium) to facilitate the opening of the graves. The zombies will then flow to the boss instead of attacking you directly.
Three Elements of Success
1. Two places to get you out of combat
2. Activate Boss but don't let him fight
3. Use Cone of Cold and Frost Nova only. Why not use Blizzard? Because you're not level 60
Finally, good luck!