In the Arena which was past of the DLC in the original you need to complete 5 waves in each of the 4 areanas. There are two on the top and two on the bottom at the back.
This can be done solo or in coop. Unlike the game, the enemies DO NOT scale to your level. In VERY HARD, the enemies are around level 37.
An easy method to complete them solo is using meat bait when they spawn and luring them in a group and then using a molatov on them.
-------------------------------
It should be noted that this achievement was glitched in the release of the game but has been patched June23, 2016.