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Edit: it's possible that lairs are randomly generated which if true renders the guide not as useful.
Best advice for finding the secret room is to kill all the enemies in each room and then check the perimeter for a room off the beaten path - before heading to the objective.
Once in the secret room, search around for the secret in the secret.
The secret usually has to be "found" by activating a panel to open a wall to reveal a new area. Thanks to HiCultureBandit for the vid
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Once inside the secret room
Skim around the walls for an extra secret little nook.
BLAMMO! Achievement unlocked
The TLDR from my previous guide is"Special Investigation: Yeti" (the mission before you actually play him).
Find the secret room in the legion lair (see video).
Explore secret room for a screter secret (where I shoot my guns at the end)
Comments are more helpful than downvotes.
Just posting this guide to help others ^_^