When you start killing him, just keep doing it. You will need three tanks for this fight. One for Anub'Rekhan, and one for each crypt guards he summons. All melee and 1-2 ranged should be on each crypt guard, and the Main Tank on Anub'Rekhan. 1-2 healers should be on each tank, especially on the one that is tanking the boss. When he casts an insect swarm, you MUST stay in the middle of the room, in the green part. Then you can go back to killing him.