Well, this is my first solution to an achievement. If there is any criticism or part of the solution is unclear, then please say so in the comments before giving me a thumbs down.
NOTE: ALL MULTIPLAYER ACHIEVEMENTS CAN BE DONE IN SQUAD MATCHES. (not guerrilla or co-op campaign achievements though obviously)
For me, this achievement description wasn't very helpful, but here is what you want to do:
FOR BOOSTERS
1.) Form two squads of 4-6 people. I would recommend 4 per squad, or 8 people total, so that you can all be in the same Xbox LIVE party.
2.) Have the party leader of each squad select Decoy in the Squad menu and search. The other party should be ready to search as soon as the first party finds an empty room.
3.) Now I'm not 100% sure about this, but this is how it worked for me. The squad who was on Team Bodark got to be the attacking team twice per round. This means the Team Bodark squad will be more likely to get the achievement first, as that squad gets to guess the objective twice per match.
For both boosting and non-boosting squads...
To get this achievement you must guess the objective right on the first try. This means that you have a 33.3333% chance of picking the correct objective. If you get it right on the first try, then it counts towards the achievement. If you don't, then it does not...
Your squad must guess the correct objective on the first try FIVE times. They do not have to be in a row. It does not matter if the other squad guesses right on the first try before your squad. After guessing the correct objective right on the first try for the fifth time, the achievement will pop.
My problem with this achievement was that I thought your squad had to complete the objective before the other squad five times. When that didn't work, I thought your squad had to complete the key objective five times in a row without the other squad completing the key objective. It turns out I am an idiot and these are incorrect methods.
In picking an objective, I recommend sticking with Objective A. (or another, just keep picking the same objective). For our group, Objective A was the key objective more often than not.
If you consider yourself lucky, you could theoretically get this achievement in five rounds by picking the correct objective five times in a row. It took us about 10-12 games of decoy after we realized what the requirements for the achievement were.
Good luck!!