Play Mission 12: Battle Of Sunaion on the Easy difficulty. After you destroy all of the turrets, you will have to make your way up some disks. At the very end of this section, there are two Hunters. You need to knock these two Hunters off the edge to get the "Worms Don't Surf" achievement. It is a lot harder than it sounds, and is easier to do in co-op. If you are playing solo, either keep sending your AI team away from you or kill them so they do not kill the Hunters or enrage them. Get the Hunters to swing at you and get near the edge so they fall off. Once one of the Hunters is near the edge, get him to lunge at you and then dash away. If he does not fall off, Spartan Charge into him to shock him, and then try again. A good reason to do this in co-op is if you fall off with the first Hunter, a teammate can try to get the other Hunter to fall off the edge. After the first Hunter falls off the edge, the second Hunter will become enraged. Just keep running around the edge to lure the Hunter near you. Stand between him and the edge, and eventually he will swing at you and fall off the edge.