First, if you don't have one already, set yourself up an Ubisoft Connect account; otherwise you can't play a Network Game which is required for this achievement and others involving ONLINE play. It's easy to do and no money is involved.
Second, go into Network Game. From here, you can either Create Match, or Join Match. Joining a match is iffy because if the server the host is playing on isn't good enough, you'll get kicked out; also, sometimes there are no matches set up, so you'll have to make one up.
I highly recommend Create Match; select your Server and # of Players, then your character and difficulty setting. I recommend Average Joe to speed things up. Once you're on the map, go to "The Main Stage" which is part 2 of Stage 3. Once you've selected this, you have to wait for people to join your game. Note: you cannot begin the match unless all your player slots are filled, and if all of the other players quit, the match is cancelled out and you'll go back to the Main Menu.
By doing "the Main Stage" you skip the early half of the Stage and do the mini-boss fight, then the boss fight with Todd. I did this both with Ramona and Wallace, and this counted to the achievement. You could play any of the levels, but I found this to be faster.