This achievement is for winning a match of versus in any mode with 5 people playing as Recruit Clayton. You can have 5 people or 3 people and 2 of them with guest accounts. You can do this in a standard match if you wish or play a private match against casual bots. Make sure everyone has Recruit Clayton character selected and you all are on the COG side. This works in any Versus mode but a private game of Warzone on Checkout for one round will be fastest. When the match is over, the achievement unlocks.
If using guest accounts, there can be problems with the host being one of the ones with a guest account. It may work for you but it may not. If you have any problems, that is one thing you can check.