I got this initially by luck but then realised it's a good strategy.
Play single player, with bots and king of the hill. The 4 computer bots tend to defend all bunched up and equally attack together, so often you will find yourself facing 4 enemy soldiers, who on medium difficulty are about as dangerous as retired Morris Dancers.
As a medium class, using the M4 just rinse them. The time between kills is quite generous so it shouldn't be a problem. I was getting Triple and Quad kills every minute and I'm no gamer pro, so with a bit of patience you will be fine.