Another easy method in addition to sic_doctors' guide works like this:
Enter splitscreen mode with two controllers. Select Sniper Fi in the Alpha Ops. As soon as the game starts lie down with the guest account. With the account that's gonna get the Trophy you'll pick up the Predator controls. Now crouch near your guest account. Use the drone to send a missile towards a bigger group of enemies. As soon as the missile is on it's way you'll pick up the second controller and knife your main account so he's down when the missile hits. If it killed at least 4 enemies...Trophy unlocked.
There's also a nice video:
Video credits to MrBlingBlingNr1