MechWarrior 5: Mercenaries

As the achievement states, this only applies to campaign or career missions - not instant action. What's not immediately clear however, is how to accomplish this.

The first thing everyone should do, is add the person or persons via the "add contact" button in the window that pops up when you go to CO-OP->JOIN. Share your invite codes between each other and add the other(s) using their codes.

The Host

Next, the host should either choose CONTINUE or LOAD from the CO-OP menu, and load the desired single player campaign save. At first, you won't notice anything different than if you were to just play single player - that's ok. Continue by doing everything you would normally do to start a contract, but make sure you have enough mechs added to your lance for the person or people you want to invite.

Once the mission loads and the game either starts doing the powering up scene in the dropship, or simply opens with you on the planet, open your pause menu, and now, on the right side of the screen you'll notice three more boxes, below the one with your gamertag, to invite people. So go ahead and add the people you want to add.

The Others

Those who want to join should be in the CO-OP menu. Wait for a red notification icon to pop up next to JOIN and then select that option. Follow the prompts, and the game will begin to load in. If you're loaded into the map, go have fun. If it kicks you back to the main menu, try restarting the game and trying again. You'll have to ask the host to resend the invite, which they can do by clicking your gamertag in the pause menu, and selecting "resend invite"

Only The Lonely

So, maybe you've come to this guide, because you're a solo player, who has done all the solo achievements, and are now left with this one little achievement keeping you from 100%. And you want to know, like I did, whether it's possible to solo it.

The answer, is kinda. There's no way to trick the game into thinking an AI player is real, so that won't work. You are going to have to co-op with an actual person, with an actual gamertag. But while the other gamertag can't be the one you're currently using, there's nothing saying the other person can't be you.

This game is currently on GamePass and GamePass cloud gaming, so all you need to do is create a new account with a new gamertag and subscribe to Gamepass Ultimate. Then, using this account, you can use any platform Gamepass or cloud gaming supports, to play a 2nd copy of the game, which you can then use to be invited to the co-op missions by your main account. The best news is that if you knock the achievement out within a day or two, and cancel your new Gamepass subscription, you'll get the $16 refunded.

The potential downside is, your co-op partner is going to be pretty much dead weight, so plan on picking easy missions, and compensating for this with the remaining two AI lancemates.