Call of Duty: Modern Warfare 2

There's only two vehicles in this game in which you can shoot from, they are the snowmobile which is featured in a few special ops and also on the mission 'cliffhangar' in the campaign. The other vehicle is boat on the last mission on the campaign called 'Endgame', here you drive a dingy and you can shoot while driving. You can choose either method to unlock the trophy. For the purposes of the guide we'll go with the boat option as there is a lot more enemies to kill. Just drive the boat slower and whenever another boat appears in front of you just shoot at them until their boat sinks. You will also want to shoot the enemies on the shoreline and on the rocks. The game is very lenient with the accuracy and you can just spray and get a kill, you don't have to aim so much. If you reach the waterfall and you haven't unlocked the trophy, restart the mission. The trophy will unlock upon killing the 20th person in a row during the mission.

Here is a video by Nextgentactics on how to do it on the spec ops mission Race, a different mission:

https://www.youtube.com/watch?v=uRKKwh9Xd7U