If you've run into an issue with getting the Together Till The End achievement/trophy or and can't get the Bozak Mask, perform the steps below, please:

1) Uninstall the Bozak DLC

2) Re-install the Bozak DLC

3) Play the DLC and be the host, invite 1 person

4) At end kill the other player with the gun (you do not need to take any damage)