How to do itJust start a private Multiplayer match in the Sewer map.
You need to get 5 enemies burning and leash or kick them into the green water on the opposite site of the flamethrower thing in the center of the map.
I have done it in wave 3. I upgraded my leash 1 time and defense 1 time. Then I stood next to the water (outside the range of the fire), started the fire via the leash and pulled the enemies again via the leash trough the fire. Then I kicked them into the water (1 kick max, dont kill them in the air). I killed every "unleashable" enemy because they annoyed me, but you can ignore them.
Tip for GERMANS:Its extinguishing, like removing the fire with water.
In german its wrongly translated to "auslöschen" which means "killing".
SideTip for other achievements: "Anarchy Mode" = Multiplayer Mode
My short Vid (in german, but you can just mute it)