Have all other players run out of life in the same area at the same time. Clear the enemies to not be interrupted and press
over their bodies. Then you press
again on the heart in the timing game like a defibrilator. You can also mash the
button to revive someone quickly.
In Local Co-op, sign in 4 controllers. Have all the controllers signed into a profile - can use guests, silver or gold accounts. Have everyone select a main character knight (one of the colored knights: Blue, Red, Green or Orange Knight).
In online Multiplayer, you can do the same tactic of having all players go down, clear the enemies, and do the heart timing game on each of them. You can sign in guests to fill missing slots as long as you have four players.
NOTE: You only need to revive each of the different players once while playing the campaign, but it is easier to do everyone all at once in one area.Extra note for Win 10 people:AKXOne said:
If any of you have Windows 10 on a PC or laptop, as well as 4 controllers in total (combination of Xbox One and Xbox 360 can be used), you can boost this yourself using the Xbox One Streaming feature of Windows 10.
Credit goes to Istari Phoenix as he used this method to get the local 4 player achievement in Halo MCC (Solution for Back In The Day in Halo: The Master Chief Collection).
I did it using two wireless Xbox One controllers and two wired Xbox 360 controllers connected to my laptop. I started streaming my Xbox One via the Xbox app on my laptop, signed myself in as Player 1 and then signed in 3 guests on the other controllers.
Each player was a standard knight (Orange, Green, Red and Blue) then I started a game on the Barbarian War level. I let the other 3 players get killed, cleared the mobs and then revived each player. The achievement popped as soon as the 3rd revive was complete.