The enemies in the arenas scale to the host's level.
Easiet way -
Get a level 10 character to host the game, and 3 other level 50 characters as co-op partners (one level 50 is enough, but you will go through it faster with more people) You will be able to breeze through the levels without any trouble.
its 5 rounds, each with 5 waves. so it will take while. so make sure you have around an hour in your hand, because you cant quit in the middle of an arena.
If you Join some one else's game half way through, you can still get the achievement, as long as you are there at round 5, wave 5. (so if you lag out, just join back in, it wont effect the achievement)
You get 1 skill point for finishing all 3 small arenas in play thorough 1, and another for finishing them in play through 2.
(you have to be atleast a 10/have fast travel to access the mad moxi dlc)