Aerial Unit R-21/X

When you enter Operation: Mechagon, HK-8 Aerial Oppression Unit will be patrolling above one of the first three bosses. To activate Hard Mode, you must defeat each of the first three bosses while HK-8 Aerial Oppression Unit is hovering over their area, rather than avoiding it.

During each of the first three Hard Mode boss fights, Walkie Shockie X1 robots will spawn and slowly follow random players while using Static Discharge. These robots are immune to damage but can be crowd controlled — if they reach full energy they will self-destruct, so use crowd control to prevent this.

After defeating the first three bosses in Hard Mode, you must then kill HK-8 Aerial Oppression Unit. In Hard Mode you will not have enough time to hack the towers before Annihilation finishes casting, so as soon as Tank Buster MK2 dies, run back to Rustbolt and position yourself in the inn area on top of Niru Datagear. When the Annihilation cast finishes, jump off the Rustbolt platform toward the boss and close the distance immediately — if done correctly the boss will begin chasing you instead of resetting.

Completing the dungeon in Hard Mode grants the achievement Keep DPS-ing and Nobody Explodes, after which Aerial Unit R-21/X is a guaranteed drop from King Mechagon.