Throughout the fight, Trilliax will spawn Toxic Slices of cake around the room of the encounter. Normally, these should be picked up to prevent raid-wide damage. For the achievement, however, you must ensure that, by the time Trilliax is defeated, the raid as a whole has consumed no more than 20 slices.
Note that the achievement is party-wide, meaning that no more than 20 slices must be consumed by the entire raid, as opposed to each individual player.
- Patch 7.0.3 (2016-07-19): Added.