I'm trying to complete ACIII missions on AC Initiates website. Missions are usually easy (like completing 1 multiplayer session of Domination mode. The thing is there are a few players who still play this game online and it's difficult to complete this mission.
So, anyone who wants to help me feel free to add me on UPlay.
I had this same issue so let me share how I solved it:
Make a friend, as in literally connect to them through Uplay, and invite them to a game of domination. This will fill out half of the required number of people to start a domination match and that will put your match on a high priority for people looking for any team based matchup. I spent literally hours trying to find a domination match solo with no luck. Tried this once and got it immediately.
Another thing you can try is to post either here, the multiplayer forums, or the AC:Initiates thread about the particular mission to see if people are willing to help. If you coordinate a time and at least four people want to play a round you'll have it done in no time. Feel free to message me if I miss that thread as I'll be happy to help if I can.