Our setup was:
GLA x2
CON x3
DD/THM x3

You can probably do it with 7 people or less tbh but we can do it with 8 even tho we had 9 people at the time :P

Main tank holds the NM, need at least 2 CONs curing the tank.

The other GLA off tanking the adds while DDs/THMs finish them off one by one or a couple at a time.

Depends on mana we would wait in between waves but if all mages have at least 1k + on mp then its safe to go ahead and start the next wave.

Once each wave pops, nobody will be attacking the main NM except for the main tank.

Kill adds and then goto the next wave..and so on.

After 3rd wave, just zerg the NM - zerg through 4th wave, so ignore the drakes when the pop.

Start off with 2 brothers (LNCs) and the NM,

2nd wave - 4 sisters (ARCs)

3rd wave - 4 Scrivners - (THMs)

Last wave - 4 Drakes


You can probably do this zerg style with more people or a different setup but this is our strat for the Zahar'ak Boss ^^

Heres the video:
http://youtu.be/7ElKg8RYBqo