Turning the Tide

On the platforms in the south and the north there are 5 untargetable mobs, which spawn when you begin the fight. In order to kill them, you have to move the Boss close to the wall and group up right before barrage is about to happen. If done correctly, the mobs will die from the barrage damage in one cast.

Easy Achievement for Level 120 Battle for Azeroth and beyond...

1) Normal Difficulty.

2) Begin fight, then make sure that the Hellfire Guardians are standing on top of the walls on the north and south side, there should be 5 on each wall, for a total of 10.

3) Situate yourself between the boss and the Guardians and eventually boss will cast Barrage. I had a pet, so it was easy to keep aggro on boss and tell pet where to stand. The barrage is what kills the guardians on the wall.

4) Get the five down on the one side, and then run to the other side and get the 5 down there. The achievement will turn white when it's complete.

5) Nuke down boss after all Guardians killed.

6) Profit. Achievement Completed.

Crazy easy at max level. Just run into the boss, no need to attack, then bring it over to the Northern wall. It'll cast Barrage which will kill the guys on top of the walls. Repeat until one side is done, then head over to the Southern wall and rinse and repeat!