Defying Gravity

DId it with 9 last night however...

this requires at least 10 people.

Don't make our mistake.

Important to note that you don't have to wait for Dimensius to choose each individual player in your raid with the debuff or anything like that (in fact that would be impossible). Simply have players stand in the Reverse Gravity AOE circles around the chosen players - everyone within the circle gets knocked in the air.

You can quite easily have your entire raid (minus the 2 players with the initial Excess Mass) jump into the first set of Reverse Gravity circles and have everyone affected at once. Just stack them all up and you can pull them to the ground together without issue.

Then on the next cycle after Devour, make sure a different 2 players pick up Excess Mass so that your original two Excess Mass players can get thrown up as well. By the third cast of Reverse Gravity, you should have the entire raid done and can push it the boss into Phase 2, job done.

Adding, because I didn't see anyone mentioning it. While you can do all the other achievements with 8 people, this one only triggered by 10 for some reason. We all went up, but the achievement stayed red.