Actually, you only need to initially kill Zathra twice before you kill anything else. Gynok revives one mob himself which is where the timer comes into play. However, Zathra also is the only add that revives other adds. If you dps the adds initially to 50% and then concentrate on Zathra until he dies twice the AOE's should bring the rest of the adds down far enough where they will all die shortly after Zathra dies for the second time.
There is also an effect cast on players throughout the raid fight. This effect will turn your screen blue. "Mark of the Gnollslayer" is what it is called. With this effect you need to be standing within 5 meters of Gynok before it expires on you otherwise you will blow up your raid. It is important not to DPS gynok with this effect on you because you can run the risk of getting knocked back by his damage shield.
Setup a cure rotation for four of your healers for the curses placed on your raiders by the Marn Brothers. The curse comes every 30 seconds and there will be 2 at a time. The curse can come within 5 seconds of eachother or sometimes more but they will come. Cure curse is a 60 second recast so you need 4 healers for a proper rotation. If you need one person to call out who is curing who then do so. Keep up on the cure rotation.