Any Map (I Used Beachhead)
This took me 5 minutes in firefight rather than grouping up in a custom multiplayer match. From the start of the match take out the first few waves using the rocket launcher. Roughly the 3rd wave will drop off Elites.
Get behind the Elites and hold B, this will assassinate the elite if you are positioned correctly. Do it 10 times and the achievement unlocks.
You can change the firefight settings to give you unlimited health to make this easier