Speak with Googegguta just outside the entrance in the Thundering Steppes at ( -93, -3, 1011 ) /waypoint -93, -3, 1011. Complete the dialogue with him to gain the ability to choose this instance in the 'Select Destination' tab for Cove of Decay. Every time you zone out of the instance you must speak with Googegguta again.
Position the raid in the water, but as close to the tents by the beach as possible and kill the 52 ^^ heroic fish in the water (grouped encounter). Immediately after the last fish dies, Rognog and his crabs (heroic-epics) will charge the raid.
Rognog has an ApE knockback that will send everyone flying if they do not have their back against something. At about 25% he will heal to 100% and become a x4.