Obtain 20 000 points in the Singapore mission in the Campaign.

This is probably the hardest assignment in the game. Singapore is the only mission in which I failed to score the requisite points during my initial Hard playthrough. 20,000 points requires several multi-kills and they are difficult to pull off in this mission. I suggest alleviating yourself of a headache and not attempting to complete this assignment on your first Hard playthrough. Instead, try to kill 15 enemies with the RPG and unlock War turtle.

After completing the mission on Hard you can replay it on Easy, where the multi-kills will not be so problematic. Just employing the strategies from the previous missions should be more than enough to get you the points you need.