Under the Fist Icon on the Star chart, shows the current Invasions. Complete each pint of these Mission 3 times to get the chance for Battle pay.
In the case of Infestation Outbreaks, It may be possible to get all 5 Missions right next to each other.
Payment is only given out when your side wins. The Achievement will pop upon opening the 5th payment message.
If you run the boss node, you will fight Phorid, which can drop the parts to Nyx warframe.