First of all using 3 extra controllers to get some dummy accounts into the game speeds everything up. Now place them at the egde and use your weapon with
to knock them out of the map. I activated the achievement tracker to see my progress during a match. Maybe you want to do the same.
I played the
Chimney level to unlock the
trashbag achievement. This weapon looks like a small black bag. Which weapon you get is random and sometimes it takes a long time until the one you need appears again, which is pretty annoying. You can hold 2 weapons. If you have 2 you don't need for an achievement, simply kill yourself and respawn with empty hands.
Please note there are maybe other places to get the stated item above, but that's the one place i used. If you have other suggestions or an improvement feel free to leave a comment. I know my solution isn't perfect, but i found no guides and hope it still helps out somehow.