To get this achievement you have to kill player with Flametrooper kit with any melee gun(included bayonet charge). This kit appers in wodden box around the map. If you want boost it check locations of this kit down below on three new maps.It is possible to get on all maps not only on Apocalypse. Look at the comments for other locations.
I believe there is only one box on each map with this kit.
Click on pictures for better resolution and zoom.
EDIT: Thanks to Jonny1872 for new location on Prise De TahureCaporettoFlag B:
On left side of destroyed house if you are facing flag APasschendaeleFlag B:
Inside a bunker next to the flagRiver SommeFlag E and F:
Between these two flags, behind the bridge with two red wagons if you are facing flag DPrise De TahureFlag C:
In the middle of the square behind the statue