Activating this mission will change your squad to the beginning heroes- Hardtack, Hollywood, and Fortune. It'll even put them in matching outfits. Drive to the LEGION lair and scan for the entrance, in this case a bicycle. Hack it to gain access.
The lair itself is pretty standard- the twist is that you can't switch party members. Each hero will take their own room, and keep killing LEGION goons until the meter in the top left of the screen fills. The first room is Hollywood's, then Hardtack's, then Fortune's. Tactics will vary by how much you've leveled the heroes at this point, but you can always reach a slow-and-steady victory by hanging out near the entrance and letting the enemy come to you.
When you get toward the end of your meter, a commander will appear. Killing them will generally put you over the top.
After completing Fortune's room, you'll gain access to the whole team again, for a final room with shielded troopers and the Pride Captain, a Repairing Slagbot.
Chew through the mooks, then use one of the tall consoles to play ring-around-the-rosie with the slag bot. This lets you evade his beam attacks, and get in position to attack the weak spot on his back.
Once downed he drops some impressive loot, including the mission macguffin. Pick it up for a cutscene, and mission completion. Note that this earns all threee squaddies their Franchise Force skins. Equipping all the skins on all three characters is worth an achievement.