This is the easiest achievement in the Bloodbath arena and shouldn't take more than 10 minutes to get.
Advised character level = 10
First of all you will need the Bloodbath Arena DLC priced at 800MSP. The Bloodbath arena is located in the Resort section of the island near the lighthouse, instead of driving forwards through the gates towards the lighthouse, turn left and travel forwards until you reach a tunnel. At the beginning of the tunnel there will be a door which you can go through. (To speed this process up, once you've downloaded the DLC, you can fast travel here from the lifeguard centre or the lighthouse once you've gotten that far.
Once inside, there will be a short cutscene introducing you to the area, after it's ended you can take a few minutes to look around (There is a blueprint and an island article to be found). At the back of the area, there will be four doors named; A, B, C and D. Each arena varies in difficulty but you won't really see the difference until after round 5.
To begin enter door A, you will now appear in a very small arena with a giant hole in the middle - tread carefully. The rounds will now start and 4 very weak zombies (walkers) will come towards you. Punching and kicking works and will kill the first 5 waves of enemies in a flash. Once round 6 begins you can either choose to stay and continue fighting or you can leave and go to the next door (B, C and D)
Each arena is different and will change with difficulty, but if you have completed a few missions and most of Act 1, then this achievement shall bring you no trouble.
Note - there are workbenches in each arena and a short amount of time to fix and upgrade weapons between rounds
(Be sure to pick up the blueprint that lies within each arena!)
- To those who say I've overcomplicated this or explained it too much, this guide is aimed at people who are new to the game and have only recently downloaded the Bloodbath DLC!