"Comrades" is a new quest available in the multiplayer expansion, where you have the choice to play as 1 of the 4 main characters from the story. You simply need to replay this mission 4 different times, each with a different character, to unlock the achievement. To start the quest, talk to Monica like you would with any other repeatable multiplayer quest.
The mission consists of:
2 regular enemy waves.
2 waves defending a truck along with a level 48 Nagaragi boss.
2 waves of defending a building along with 2 level 33 and level 45 mech bosses.
5 waves of basic enemies while moving towards the final area.
Final boss battle with a level 56 behemoth.
It seems like your level is fixed, so don't worry about grinding beforehand (Let me know if theres a way to increase your level).
It wasn't exactly tough, just time consuming as it takes around 30 mins to complete per character, so be patient. It also doesn't need to be in one sitting, so feel free to take your time completing this with each character.