You have to do 10 kills with Engineers, while maintaining a multiplier. Therefore, you should use maps with lots of enemies. Here are a few choices:
The start of the mission, after blowing up the 1st bunker, has enough enemies. Use the shotgun on soldiers, and quickly switch to the RPG when a truck appears.
You can also use the same tactic in 2 other missions (forgot the names though...) : the mission that looks like the invasion of Normandy, where you start in a boat and must capture the hill, and the mission with tank on tank action in a snowy environment.