The Reef Guardian will split into smaller copies of itself. In total you will have to deal with the large version you start with, 2 medium and 2 small copies. After some time the guardians will walk over to one of the smaller holes at the outside edge of the room and become immune.
You will notice that there are 6 small holes placed around the room, each with a different symbol. There is also a large hole in the middle. When a guardian becomes immune a dps or two will need to jump down in the large hole in the middle and find their way to the reef heart directly underneath the guardian.
As you jump down as a DPS you will see 3 whirlpools which you can synergize with to take you in the right direction. You can also look for the symbol which is glowing next to the entrance where a channeling guardian is. Each path covers two of the smaller holes and you may need to continue in to get to the reef heart which you will need to destroy. You have 60 seconds to destroy a reef heart which has a total of 776k health. If you fail to destroy the reef heart in time the ceiling will collapse and the entire group will wipe. The crab does not need to be killed but you need to avoid the cone from the heart and the crabs attack, a self heal is recommended. Once done a player can use the synergy to get back up again. The player will be infected with a parasite for 45 seconds, trying to go down with the parasite will instantly kill the player.
The reef guardian will typically go to channel by the closest hole so keeping the boss next to the hole with the crown in the beginning seems to be optimal. Having 3 groups for the reefs is usually enough to give players time to clear the parasite. With practice most dps should be able to solo a reef heart but if you are new to the fight I recommend sending 2 players per reef.
That’s the main mechanic of the fight covered.
Each guardian has an acid cone ability which will lay down poison puddles under anyone in the cone. It is important that tanks keep the cone away from the dps while moving out of the AOE on the ground. Staying in the AOE will give acid reflux stacks which increases the amount of damage taken from the acid cone. Stacks last for 5 seconds but refresh when a new stack is received. It is therefore good to avoid having too many reef guardians up and to split them between the tanks.
When a reef guardian is channeling the entire group will get hit by lightning bolts and will take stacking damage, it is recommended to clear these at 7 stacks. Tanks should try to clear before 15 stacks. There will be a line of lightning indicating the areas which can be used for cleansing. The cleansing areas have poison stacks and I would recommend to quickly go in to clear lightning stacks and then back out again. Do not let the group stand in the poison area as the flowers will spawn increasing amounts of poison.
The reef guardian will spawn the first medium guardian at 80%. Groups tend to either move the largest reef guardian away while killing the medium and small copies before doing damage to the large guardian again. This is to always have a maximum of three guardians. If the group has high enough dps you may be able to keep all the guardians stacked and kill a few of them before they move to do their channel ability.
Summary
That’s all for this guide. I have covered the basics but there is a lot more to the fight if you would like to know more details. If you are interested in learning more I would recommend these guides below: