From the main menu select Multiplayer
or Warzone Firefight
from the Playlist and press
During a Warzone
match, 3 Legendary Bosses will appear. Their position will be shown by a yellow marker. You have to be the one to land the killing blow or do a significant amount of the damage. Using heavy weapons or a vehicle will give you a much better chance of doing a significant amount of damage, or your shot doing enough damage to kill the boss. An alternative approach is to use a weapon with a high rate of fire.
During a Warzone Firefight
match there will be a lot more enemies classed as Legendary Bosses, not to mention the new Mythic Bosses. Since there are only 7 other players in Firefight (and they are all on your team) compared to 23 in standard Warzone, you will have a much greater chance of getting this achievement in this playlist.
There are two other Warzone related achievements: