Infiltrate the Underground Base

The enemies will already know you're inside the base, keeping them in constant caution status. Be sure to not elevate that level by keeping your MK.22 and other silenced weapons handy. Tranq the first groupf of guards to the right and enter the elevator to emerge in the underground passage.

You can shimmy across the ledges to avoid detection for the patrolling enemies. The passage leading out of the area is guarded by shield wielding soldiers. While they are deadly during frontal assaults, they are still hopeless to CQC attacks from behind.

When the zone is safe, make your way to the east and open the shutter door with repeated taps of the action button.