This is pretty straight forward... you'll get this at the end of the DLC. Yes you guessed right: after defeating the "final boss".
I don't want to spoil too much, but hints that might help you against the boss (i did it with my infiltrator on insane):
1) you need to focus on those orbs that keep moving to the columns. Doesn't matter what else is going on, first stop those orbs.
ARC PROJECTOR is your friend (so equip it when entering the Atlas Station) as it will hit several orbs at same time and with two shots you can kill 2 or more orbs, if you hit them when they are still close to the core.
2) after destroying those orbs, the VI core will be exposed for a short period for you to shoot it. after certain amount of damage the fight goes back to point 1)
3) you'll get geth spawns from time to time. finish them quickly to continue focusing on the orbs.
and that's about it. it's not a hard fight. as i mentioned, i used a lvl30 infiltrator mainly using the arc projector and that SMG i got from kazumis DLC. my special skill is reave but i didn't even have to use it.
EDIT: i'd appretiate feedback from everyone, specially negative voters to know what's wrong with the solution.