The Lord of the Rings War in the North

I am not entirely certain about this one. But, I noticed when I obtained this achievement, was only with myself, a friend, and a bot. We all equally attacked an enemy, taking turns with attacking. So, try this, and let me know if it works lol!

*Edit* I think from what I have been seeing in the comments, and personal experience, I believe the first two players in the lobby might have a better chance than the 3rd person that comes into a lobby. I remember when I never got the achievement as the 3rd person into the lobby, just only when I was either the host, or the 2nd person into the lobby.

Strength of Our Alliance
Slay one enemy together with 2 other players.

This is the one achievement that requires you to play online with other players. For the simplest way to obtain this achievement you will need to do the following...

1. Get one or two other human players (we used two human players one was playing split screen).
2. Start up a new game on the normal difficulty, other difficulties will work this was the easiest.
3.
3.1. You will begin in Bree, proceed to the first enemy encounter in Fornost.
3.2. Move on to the first two enemies up the stairs. Destroy one so that you may focus on just one enemy without complications.
3.3. Next, use to block and to push the enemy into the wall.
3.4. Surround your enemy with all three players
3.5. Smack the enemy around a bit until the yellow arrow appears above his head, for the best results make sure all players hit the execution button as soon as the yellow arrow appears. You should obtain the coop/group kill execution.
4. The achievement is yours.

Be prepared to try this a few times. I found this to be the easiest way to obtain this achievement. Took me less then ten minutes to set up and try out. Hope this helps.

Also, credit to, zaingunsman, for helping with this solution.

The difficulty of this achievement is entirely in gauging your latency. The game is not forgiving in the timing of your button presses like most multiplayer titles and requires systematically adjusting unknown variables into known ones. I will explain what I mean.

What generally happens when this achievement is attempted is that all three players will immediately group around the nearest enemy, not get the group kill, and repeat ad nauseam because everyone is trying to adjust for the other two players. The goal post keeps changing because nobody is separating the variables.

To begin, start a new game on easy on a new save for everyone. Choose the person with the lowest ping and highest bandwidth to host (use sites like speedtest.net to determine this if you have to).

The host will do the countdown and will choose a consistent pace for the other two players to base their timing on. I recommend "3, 2, 1" but as long as there's no definite "go" signal for confusion you can do whatever you want. The host will choose a point in this countdown to hit his Y button every time and will never change it. I recommend directly after "1".

Now choose one of the other players. Since the host will not change his timing at all, this second player must continue to change his timing of pressing Y until them and the host can consistently coop kill. A coop kill means 2 players have successfully synchronized their Y button, so you're already mostly home when this occurs.

Similar to player 2, the third guy will now join in and the same process repeats. The host and player 2 will not change their method or timing at all because they're not the problem variable. Player 3 must coordinate timing with the other two and not the other way around, but fortunately the grandiose coop kill animation gives them a good frame of reference for timing.

Once player 3 synchronizes, the achievement is immediately unlocked by a group kill.