I had time to keep resetting the “Into Darkness” mission to try for this achievement today, and managed to complete it. Here are my findings…
- If you make a mistake, you can reset the achievement status by leaving the instance (by walking out, or logging out).
- Keypads must be within the light radius of a flare to open closed gates. Be careful when throwing the flare to not hit nearby enemies, as hitting enemies will aggro them.
- Enemies standing in front of closed gates can be snuck past if you’re not within the light radius of a flare.
- The first boss can be bypassed by running straight to the objective on the bottom floor.
- After the mission objective updates on the bottom floor, four enemies will spawn and automatically aggro you. You’ll need to run all the way back up to the top floor, with the first boss and other enemies in tow. Use a flare to stun enemies when you need to activate a keypad to open a closed gate. Having healing abilities slotted will be very helpful. (I don’t know if there’s a more elegant strategy for this part.)
- Once you’ve returned to the top floor, you’ll get stunned by a flashbang grenade. All other enemies will de-aggro at this point, and you’ll be able to fight the final boss one-on-one.
- The achievement will be complete once you defeat the final boss, so you can
safelytheoretically go back for legends afterwards.