All three of these are rewards for completing the "Echoes of the Past" side mission that comes the DLC, so you cannot unlock this achievement without first unlocking
Once you have completed the side mission, go to a base camp and equip the Umbrage 3-80 Shade
rifle and the Brocken Shadow
outfit. Note that both of these are variations on similarly named items that were bundled with the DLC, so be sure that you are equipping these exact items, not their more vanilla counterparts.
The Grenadier skill is already learned; you don't have to do anything at the base camp to make it happen. If you need to check this, hit
and tab over to your Survival Guide, then scroll down to "Skills" near the bottom (not "Skills: Warrior", etc., just "Skills"), and you will see it.
Once you have equipped the rifle and outfit, the achievement will unlock.