Assassin's Creed Unity

So this is still achievable (Got this in August 2022) even with Ubisoft’s poor handling of the server error. You still won’t be able to use the ‘Play together’ option in the menu, but mission matchmaking (for some reason) still works.

EDIT; Even this solution doesn’t work 100% of the time, the server issue is just too unpredictable. It is a lot more reliable than any other method I’ve tried though.

GO TO any co-op marker (Brown Icons on the map) and start a public match. Trying to activate a brown icon from the map won’t work, you’ll have to walk up the assassin. The matchmaking still seems to be able to find partners.

Voice chat is always the best way to co-ordinate your kills. Otherwise, the next best strategy is to keep watch of your partner and let them take the initiative. The sync kills don’t need to be perfect, there’s a second or two of leeway.

Don’t have to do this in one match, the 10 count is cumulative across missions.

If you start the match alone, or your partner quits out, hang around and the matchmaking will usually find a replacement soon-ish. This may be down to an increase in players now that Unity has been added to PS+

You have to simultaneously perform kills with another player(s) during co-op missions. However, Ubisoft has made this basically unobtainable because the servers are being turned off for the game and you can’t connect to play online for these multiplayer missions. They also cannot be done as local multiplayer.

The Xbox Achievement post recommended reporting this to the Better Business Bureau as was done with Activision for an unobtainable COD trophy/achievement.