WoWDB

The small adds that spawn from the DPS completing their challenges throw little Unleashed Corruption orbs at the boss which must be avoided t to complete the achievement.  You can either intercept these, or just prevent them from occurring.  The latter is the easiest strategy, especially if you are doing this achievement on Flex mode.

The simplest way to complete this achievement to just avoid sending any DPS into their trials.  This will prevent little adds from spawning entirely, and you will not need to worry about intercepting the unleashed corruptions as a result.  Send only the tanks and healers to their trials instead and have the DPS tunnel the boss and big adds that spawn sub 50%

Alternatively, you can make sure you have one person assigned to stand in front of each little add to make sure that they are between the add and the boss, so any orbs thrown hit them before the boss.  This can be difficult, especially when the cutter is coming out and people are required to move, and can be messed up by something as minor as not getting to a new add fast enough, or two people running to the same add and leaving another untouched.

The achievement name is a possible reference to Kologarn from the Ulduar raid. When starting the encounter, he yells, "None shall pass!"