Getting frustrated with this commendation. Anyone else who still needs this want to try and farm it?
The easiest way to close this commendation is to play castle wars and hit the Mongoose with a plasma grenade as they try to get to your castle. I hate that game mode but played it exclusively (when available) for like two weeks to get my last forty kills.
Castle Wars is the best playlist for Mongoose kills. Depending on the variant of mongeese on the map, a few sword swings are necessary to deal enough damage to the mongoose before throwing the sticky nade. If you are really focused you can get 3 kills a game.