another achievment best completed in a local Versus match with two controllers. set your main gamertag to a survivor and the guest to the infected.
go to no mercy episode 4 in the the hospital. from here just open the door from the saferoom so you can spawn your guest account. he will always come up as a smoker first, so kill him and then wait for it to respawn as a boomer.
position your boomer around a corner in the hallway so that you are the first one to encounter and kill him, before the AI survivors get the chance.
have your main character walk around the corner and quickly give the boomer a punch using the left trigger. take a few steps back quickly and then open fire using an auto shotgun.