Batman: Arkham Origins

In order to get this achievement your team must be at 0 reinforcements and you must be the last survivor at your team.
Then you need to kill etheir Batman or Robin and your entire team will ressurect and you get yourself an achievement.
Have to be done in a full 8-player server.

All of the online achievements in this game can be done in Private Matches. The multiplayer side of the game is on Disc 2 of the Retail copy of the game. No downloading or installation is required.

This achievement has to be done in the Invisible Predator Online game mode. You will not get this achievement in the Hunter, Hunted game mode.

Lobby Setup

For this achievement you will need a total of 8 players including yourself. You cannot do this achievement with a smaller group. If you have dummy accounts, that might be a recommendation to ease the congestion.

Whoever is the host needs to go into "Create Private Game" from the Main Menu and make sure Mode is Inv Pred Online (Invisible Predator Online) and the recommended map is Chemical Plant. Keep "Map Rotation" Off and then click "Start Game".

Here you need to use your and invite your group into the game until it is full with 8 people. Once you have 8 people you can start the match. If anyone get's disconnected during the match, the match will continue but they cannot return to the game as invites will be disabled.

Boosting Method

I recommend you work on this achievement as soon as possible. In Invisible Predator Online the whole team has 30 lives with the exception of the Heroes. Everyone will be placed in 3 different teams. 3 will be on the Joker Team. 3 will be on the Bane Team and 2 will be on the Heroes Team (Batman & Robin).
Only the Joker Team & The Bane Team can go for this achievement. Anyone playing as Batman or Robin cannot go for the achievement in the match, so you will have to do some rotating in order to get everyone a turn in going for the achievement.

Everyone needs to meet up somewhere on the map and then decide which team is going to go for the achievement first. Once decided that team needs to get their lives reduced down to 0. This is easily done by getting the opponents to kill them many times. Once the lives are down to 0, everyone on that team will only have 1 life each. Decide then who wants the achievement within that team and get them to stay away from the gunfire. Once decided get the 2 people on that team who are not going for it yet to die and they will not be able to respawn. Make sure co-ordination is applied because the opposite team could kill the wrong person.
Now the lonesome player on that team needs to find either Batman or Robin and then they need to kill one of them. Doing so will respawn your former 2 members of your team with 1 life each and the survivor will get the achievement.
You can now do the same for the other 2 members of the team if they need the achievement by killing the other two players and keeping that person alive to kill Batman or Robin.

You can do the same for the opposite team by reducing the lives down to 0 and then following the methods applied will help them get the achievement too. But be-aware that this game mode is 15 minutes and you may not have enough time to get everyone done in one match.

Credit towards my friend tornprince2012, there is a method that can be applied of you want to have certain people on certain teams before your match loads up. This applies throughout the multiplayer of the game.

From the main menu, if you go to "Options" and then select "Game Options" you should be given the option to choose your "Faction Preference". Originally it says None, but you can change this to either "Joker" or "Bane" so that you have a high chance in joining either of the two teams you wish to be on. If you are wanting to be "Batman/Robin", simply keep this as None and choose "Hero Opt-Out" as "No". Get everyone else to choose "Yes" and this will give you a high chance in becoming "Batman/Robin" for the round you are playing on. However even though you have chosen these team members doesn't 100% guarantee you will be on that team. In addition at the end of the round, people can still "Opt Out" & "Opt In" if they want to become "Batman/Robin".

Good Luck!

I just got this 30 minutes ago. Contrary to what the Achievement description reads you do NOT need a full 8 player lobby. My session was working on Predator Paragon when one of ours quit unexpectedly. On a lark, we decided to try for this Achievement and it WORKED. We had 7 players, 2 Heroes, 2 Bane, and 3 Jokers. We simply alternated each round and let the Joker team get the Achievement.

I don't know if it can be done with just six, but my group got it with 7, so it IS possible.

Must be done in a lobby full of players. When you're teams reinforcements run out try to stay away from the opposing team and since the Heroes have detective mode they will try to hunt you down since you're isolated. be cautious and take them out as soon as you see them and you should earn this achievement without any trouble
Not really hard especially if you're joker's thug since you can plant a proximity bomb with D-pad left ( hold down a small area and plant it close by). Have fun!