You must finish the 3th mission on Contract or higher. You can obtain this achievement by either playing with a co-op partner or an AI partner.
You have to remember in Contractor mode your enemies will hit harder, take more damage before they go down and your partner will have less time to revive you then in Casual mode.
I recommend playing with a second person since it makes it easier however this achievement is still fairly easy playing with a AI partner.
If your having trouble:
- Take your time. Take cover as much as possible and conserve ammo. You more distance you have the better. Sniping is a great way to play in more open areas. Don't rush out and progress until you have cleared the area out first.
- Work with your partner (even if he is AI). Cover him when he/she needs it. Try not to get into areas where your taking on allot of fire, you may need to do some reviving and heavy fire makes that nearly impossible.
- There are TONS of Mission 3 walkthroughs on YouTube. There can be upto 9+ videos per mission so I did not post them here :)