Single Player: Kill 20 enemies with the Microwave cannon
You'll come across this gun [X-43 MIKE] multiple times during the campaign. Here's what it looks like:
Here are a few spots where you can find the gun:
When working your way through the subway, it'll be on the ground in the middle of loads of ammo. This is just before you blow the rubble up to make a new path and with a fair few Swarm CEPH running about.
When you finally meet Hargreaves and he reveals himself, walk into his room and look in the machine next to him on his right. The gun will be there.
Later on after meeting Hargreave's, you'll finally make it onto the bridge after going up 2 lifts. Walk past the destroyed truck on your left, over the small barrier and it'll be on the floor next to the ammo crate.
After the collapsed bridge scene and you finally make it to land again. The gun will be on the floor to your left just behind the APC.
When making your way to Central Park you'll get split up from the gang when your vehicle gets destroyed. Work your way past the initial group of CEPH and head into the building. Get in the lift which will take you to the next floor. When you leave the lift, look to your left and the gun will be on the ground next to the fallen vending machine.
Later on, continuing from the point above, you'll need to descend the building you're in. Once you jump out of it you'll be on a rooftop and across on the other building below, you'll see 2 small greenhouses. In front of the one on the right hand side is a stair case leading up into a bigger destroyed greenhouse. In here, next to the ammo crate on the bottom floor, is the gun.
During the Central Park mission, you'll come to a part early on when the whole area will be covered with CEPH. Once you've passed them, you'll go up some rounded marble like stairs which has a dome in the middle. Kill 2 CEPH at the top and head toward your objective. You'll see 2 trailers on the edge of the cliff to your right. The sliver one is slightly open and has a barrel inside. Shoot the barrel to blow it open and inside will be the gun. This is in the same area as the last Souvenir.
You should have no trouble getting 20 kills with this gun. Just use it until the ammo runs out and then pick it up later on and finish off the remaining kills. Your best bet is to use single fire shots at the normal CEPH. It should take about 3 to 4 hits to kill with just 4 bullets (unless you get a headshot). Don't waste the ammo on the Tank CEPH though, as they still take a lot to kill, so its a waste of ammo.
Very funny achievement pic to go with the Microwave Cannon name. Popcorn anyone? Hope the weapon blows up enemies like the scene from "Gremlins" where the one gets stuck in the micro then finally bursts like a swollen tic.
This gun is awesome, although it's not called the microwave cannon in the game. It's something like Mike X-43 or something. It's the best gun in the game. You just aim it an enemy and blast, watch their insides burst out in a pile of goo.