Pretty easy - go to anarchy mode, mini city map. there will be 2 helicopters going in circles up in the air.
just play until you can upgrade your leash with the thumper (you need 3500 points - it took me the first 2 rounds) and then just stand next to the dinosaur thingy and watch your spawn. as soon as the enemies appear thump them into the air (press LB and hold it, i had 3 of them up in the air) and the helicopters will fly by and kill them.