If a game is dead, then you have to either give up on the multiplayer achievements or somehow get a group to help you get the achievements, both of which suck.
i hate it when i can't get every achievement in a game bc at least one of them either requires dlc that i don't have, or is literally impossible, at least in my case (e.g, Dragged Below in RoR2 needing SoTV, or Not Bad in Nuclear Throne being impossible on console due to Dailies and Weeklies not existing on console)