What the achievement asks
Defeat the Awakened Phalanx in Awakening The Machine. Ten points.
Where the scenario is
Awakening The Machine is a small solo scenario in Gundargaz, in The Ringing Deeps. Talk to Speaker Kuldas there to queue for it.
Pick up Gearing Up for Trouble from Gnawbles first. It is in the same place, it costs nothing, and you may as well have the quest running while you do the scenario anyway.
What happens inside
Twenty waves of enemies come at you, and you are protecting a Keeper who must survive. The twentieth wave is the Awakened Phalanx, and killing it is the achievement.
The first ten waves are not a threat to a character in current gear. It tightens up around waves fifteen to twenty, and the boss itself is reported as easier than the waves just before it.
Three things that make it comfortable
Interrupt everything that channels. Enemies casting a channelled ability die outright the moment they are interrupted, and anything counts: a kick, a stun, a silence, a knockback. That is a large part of the difficulty removed by one habit.
Stand in the green pools. From around wave eleven, medbots start dropping green vials on the floor when they die. Running over one heals both you and the Keeper, sometimes for a third of his health. They despawn after a while, so you can carry one into the next wave but you cannot stockpile them.
Take the breaks. There is a pause every five waves, and it has no timer. Sit down, let your cooldowns come back, and only talk to the Keeper when you are ready. Players have waited out full cooldowns at wave fifteen without being pushed into the next one.
The one way to lose is to let the Keeper die. Pull enemies onto yourself rather than letting them reach him, and use the green pools on him rather than saving them for yourself.
Where it fits
One of the pieces of You Xal Not Pass, which rewards the Shadow of Doubt mount and is part of Worldsoul-Searching, the War Within meta.