So does anyone know what to do with this achievement? I've been to the Colosseum of the Old Ways in Artaeum, but nothing can be done and I've completed the main storyline. It says PvP, but is it really a PvP thing? If so how do I finish this achievement?
Yes, it is technically PvP. You'll need to win five one-on-one duels in that little arena. Wins for this achievement will also give you progress towards the general Duelist achievement counter as well.
You need to approach another player (not an NPC), interact with them, and select "invite to duel." There will be a five-second countdown, and then you and the other player will be able to attack each other. No other players or NPCs can be hurt during the fight. It will end when one player either is killed or forfeits by stepping of bounds. The boundary circle is fairly generous; if you start the duel in the middle of the "Colosseum," then just stay within the arena and you shouldn't have a problem.
If you're really averse to duelling, you could find a friend or guildmate (especially one who is equally averse!) and have some fun with it until you both get the achievement. It shouldn't take long to get to five.
ETA: Yes, I have the achievement. You can defeat the same person five times to get it. There's really no catch except that it's location-specific.