This needs to be dispelled asap once everyone spreads a bit. THe boss will apply Putrid Water debuff to 2 random players. Note that the Cannon causes Line of Sight, so have everyone aware of this as they can interrupt incoming heals/dispels. Once there is no more Demolishing Terror, have everyone focus fire the Gripping Terror (until another Demolisher Terror appears, then switch to Demolisher Terror again) Do not finish casting/actions unless it is instant, move asap. Note that you need to start moving the moment you see the area that will be bombarded.
Every single damage avoided will help the healer. Throughout encounter, everyone need to prioritise moving out of Call of the Deep (AoE water attack). When there is no one meleeing it, it will do heavy AoE to everyone in party regardless where they are located. Have the tank engage and all dps focus fire and kill the Demolishing Terror. There are 3 platforms where the fight will be and here is the optimal execution where we managed to kill the boss without losing anyone. I defeated this in Mythic group as healer at 303 item level (basically quest gears).
