Slayer Gates are challenging encounters that pit you against waves of enemies in tiny arenas. These can be challenging, long fights that will drain your ammo, health, Extra Lives, and armor if you are not prepared. Completing these fights will award you with an Empyrean Key which will unlock you the
Unmaykr weapon which is an alternate BFG 10k like gun.
Below are collectible guides for 6 levels but I will highlight the Slayer Key locations for this particular achievement and show the gate locations.
Mission 2: Exultia4:52 - Slayer Gate Key
Mission 3: Cultist Base6:38 - Slayer Key
Mission 5: Super Gore Nest8:45 - Slayer Key
Mission 6: Arc Complex1:00 - Slayer Key
Mission 7: Mars Core6:05 - Slayer Key
Mission 9: Taras Nabad5:31 - Slayer Key