The minimum level I would recommend doing this is 27 (as that's what I did and it became rather challenging)
As you cannot unlock the arena before completing the story quest 'The Crocodiles Jaws' and you fight in it as part of said quest, there is no way you won't unlock its fast travel. Simply go into the map and hold
to go into the arena sub menu.
The are two events (The Gallic Brothers LVL 28 and The Slaver LVL 30) and in each event there are 4 fights. The first three are just fighting waves of mooks and shouldn't prove to difficult in the gallic brothers event, but if you are only LVL 27 the fights in The Slaver can be quite challenging. Be sure to dodge around, strike quick and retreat, using Overpower on only the more defensible opponents.
Both the bosses are relatively easy and should topple with little effort. After you have completed all eight fights, the achievement will pop.
Let's keep cheevin' together!