Condemned 2: Bloodshot

A pipe appears in the practice arena when you have the light weapons turned on, they seem to break pretty quickly so it will take awhile to kill 50 enemies (any sort)
pipes aren't real common either so throw molotov's at a enemy for a 1 hit kill them the next guy may have a pipe.
when you run out of molotov's just restart the practice arena

As simple as the achievement states. Just rack up the kills with the pipe for this achievement, you will need to kill 50 enemies with it.

Any and all pipe kills count towards the 50, even in multiplayer.

I got this one in the Doll Factory on my second playthrough. You can also get this in the Bloodshot Fight Club in the practice arena.

Go to Bloodshot Fight Club > Practice Arena. Then choose Thugs > One at a time > Light melee > Molotov cocktails and begin.

The first guy will come down the stairs just to the right of where you start, and is almost always equipped with a pipe. Quickly grab a molotov ready for when he reaches the bottom of the stairs, then throw it. As soon as he ignites, he'll drop his weapon but he won't die immediately, giving you time to grab the pipe and kill him with it in one hit. (This counts as a pipe kill and not a molotov kill, since you finished him with the pipe).

I'd recommend restarting with the same settings and do this 50 times over, it's tedious but probably the fastest way to get it. You can always continue until you get another pipe but they're fairly scarce after this first one, and you can't guarantee how much health they will have when you pick one up.