Start Spreading the News - 35G
Finish the single player campaign on any difficulty
You need to finish the 19 missions of the game in a difficulty you want.
Recruit - Easy
Soldier - Medium
Veteran - Hard
Super-Soldier - Expert
To help you I put the name of the missions you need to complete.
In at the deep end
Second chance
Sudden Impact
Road rage
Lab rat
Gate keepers
Dead man walking
Seat of power
Dark heart
Semper fi or die
Corporate collapse
Train to catch
Unsafe heaven
Terminus
Power out
Eye of the storm
Masks off
Out of the ashes
A walk in the park
I recommend to run through the missions on Recruit. You can finish it in like 5-6 hours.