I just finished our second session for this yesterday. This is a really buggy achievement, but we managed to get a strategy going where we popped the achievement for all 3 of us that needed in 3 games in a row!
(in the German version for the record, but should work on all versions)
Have the person NOT going for the achievement host the game. The best map is
factory as it is a really small map, also make sure to set the
round preparation time to 10 seconds to speed things up. Also,
friendly fire ON for the initial match.
Have the host set preferences set to: (IMPORTANT: AFTER EVERY MATCH THESE RESET, EVERY TIME ONCE BACK IN THE LOBBY HAVE THE HOST SET THEIR PREFERENCES TO THIS AGAIN)
Squad leader YES
Tank commander NO
and other option NO
All other players have their preferences set to the default NO YES YES
GAME 1:
The Host team will always be winning, this first game have the opposite team suicide by throwing grenades at their feet (friendly fire is on to increase chances both players die quickly)
GAME 2:
(host set preferences again)
The player who is in the host's team will be going for the achievement, be sure that they have pressed
to vote for the leader to stay in command (earning them the earned in blood achievement).
In this game, in round 1 and 2 we had both the host and the other player get 1 kill each. In round 3,4,5, we had the host get both kills (thus host ends at 8 kills and player 2 at 2 kills)
Before you return to the lobby, the achievement will pop.
GAME 3:
(host set preferences again)
Next player who is going for the achievement change places to be on the host's team. Again make sure the vote to stay in command option is set at 1/1.
Now again, in round 1 and 2 both a kill, round 3-5 host all kills
before returning to the lobby another player has earned the achievement.
GAME 4:
Exactly the same as game 3.
Now for the 4th player to earn the achievement I would suggest to swap hosts and play 2 more games (needed anyway for the next player to earn the earned in blood)