All of these involve the same basic idea: jumping to idiotic locations and building a teleporter exit. Once there, build a gun. Enjoy the crying that ensues from people who get killed by it.
Jump up here.
Build a dispenser on the end and stand on it.
Jump and build a teleport exit.
Success! Now you can build the gun anywhere up along the rafters. Maximum annoyance.
Jump on the light, then towards the pipe. Build it as you fall.
Now you can build anywhere along the top of this second pipe.
While not as impressive as the others, this rooftop still nets a lot of kills.
Build the teleporter entrance for a little extra height. Then jump and build the exit.
Not much trickery here -- just jumping. Dispenser, board, alarm, wall, pipe, green box, done.