Broforce

Edit: Now that this game is on console let me write how to unlock on console.

Can do this locally or online. You just need four controllers. Once everyone is in and the level has started just press on all the controllers and everyone will flex. Achievement unlocked.

How to unlock on PC:

This is really easy to do with one controller and the keyboard. If you go into Options > Configure Controls > Setup Keyboard you can set the controls for keyboard players. The game lets you set up four different configurations on one keyboard. To make it easy, I set up the gesture command for configurations 1, 2, and 3 all right next to each other on the keyboard. Also take note of what the fire key is because that is how you sign into the game as a player. I believe the default for configs 1 and 2 are Z and T respectively, but it may be different on your game. Configuration 3 wasn't even setup so I assigned a key to fire and gesture.

To assign a different key for an action just scroll down to it and hit enter. *Press Key* will then show up where the key was before. Now assign any key that you like.

Going into the campaign, choose to play offline, it will still pop online. When you get to the lobby you will see four player spots that can be filled. Press the three different fire buttons on the keyboard and on your controller to log in four players. You need to do this kind of quick because a countdown starts once a player has logged in.

You can do this on any level, but it'll be easier to start a new game and do it on the first level. When all four players load in at the start just hit the gesture keys for the three keyboard players and on your controller for everyone to flex at the same time. It should pop immediately.