For this achievement what you have to is enter a Arena and finish and it should pop right? Well it didn't for me and looking at the in game achievement list it lists a different description then the actual achievement tile. What it lists there is "Play any arena in Co-op", the achievement isn't buggy just a confusion of achievement requirements.
All you have to actually do is just enter an arena in co-op (two to four people total) and it will unlock properly among entering the arena. I hope this helps because the other two solutions just confused me then helped me. Thanks for reading the "rant" at the beginning!
Edit: You could clear your cache and get the achievement in singleplayer how ever if you want the timestamp (if it matters) or have the GoTY edition (where its impossible to take away the update) you have to do whats listed above.
Edit 2: It looks like the achievement description got fixed YAY