Beat the game on Survivor mode.
Pretty easy compared to a lot of games from the 3rd person genre. Just upgrade your assault rifle whenever you can, buy some defence/medkit nano-machines for Dan. Use your own medkits to heal yourself when downed as your team mates don't pick up new ones when used and they can take a while to help you.
Also note that when you complete Survivor mode you will unlock No Mercy difficulty which also has an achievement linked to it.