Hit the marker and win a mission
Drive over to the warehouse end of town and scan for another LEGION lair.
The first several rooms you pass through will be completely empty, but soon you'll get one with a 'Clear the Room' objective. Wipe out all opposition with your beam weapon
Take an elevator down to another 'Clear the Room' room. Now's a good time to get used to your turret- you can heal it by firing at it, but this means you can't really shoot at enemies near or behind it. Get a feel for the space, because it can be annoying if you're not ready for the switch from harm to heal.
In the next challenge room, hack the server (Joule is a master programmer, so can bypass any hack with 'W,' though there are no hostiles here yet, so you might as well go for the XP).
Once hacked, you'll need to fend off waves and waves of enemies, including a turret. Use your own powers and MAYHEM to chew them all up.
Completing the battle automatically ends the mission, unlocking Joule as a playable character.