The new Strikeout mode is the best and easiest way to unlock this achievement.
In this mode, a cross between « king of the hill » and « team deathmatch », the two teams have « tickets » representing the amount of respawns that each team has. As you’ll be able to respawn, so will the enemies, which means it’ll be much easier to down 4 of them.
Every round lasts a few minutes, so you’ll eventually get 4 downs in one of them.