Step 1: Acquire a Mercenary Stronghold KeycardTry to find a Mercenary Camp on the map, clear out all the enemies, and loot the cache here to find the Stronghold Keycard. Another option is to buy one for 2000 credits at a Buy Station.
Step 2: Acquire a Legacy Fortress KeycardNow, open up your map to find a Mercenary Stronghold. Go there with your newly acquired keycard which will allow you to open the door. Clear out the base, a drill the safe, protecting it for a minute until it's done. Once it's done, loot the safe from the Stronghold to acquire the Legacy Fortress Keycard.
Step 3: Complete the Legacy FortressNear the top left corner of the map, you'll now notice the Legacy Fortress. It will highly recommended that you come here with some Pack-a-Punched weapons and plenty of armor. You'll want to start by taking out all the enemies and the attack helicopter outside. Then approach the building, use your Legacy keycard to open it up and clear this base as well. Watch out for plenty of tough enemies, traps, and even decoys. As you clear the base, you'll find the
Legacy Warlord in the final room. Make sure everyone on your squad hits the target to unlock the achievement, as you will need an assist at minimum.
When they are eliminated, they should drop something that can range from a self-revive to a wonder weapon. There will also be rewards that spawn in the final room.