The easiest place to do this is on level 4-1, easy difficulty using Rico. You'll have to go down a staircase and there will be a ton of enemies in your way. Just keep heading down, sprinting whenever possible, and use the machine gun to mow down anyone in your path. You also get three rockets which you should use strategically on any large groups.
It may take a couple tries (especially if you screw up a rocket and have to sit through the reloading animation), but you shouldn't have too hard of a time.