This tutorial shows you how to make a mob arena! the video explains it all! But, if you couldnt see the commands here they are! :

/ma addarena [name]             //creates a arena
/ma setup [name]                // allows you to setup the arena
/ma edit [name]                // alows you to edit it safely
/ma setclass [class name]               // sets a class with your ccurrent inventory
/ma j* [arena name]                    // Joins the arena

*It can be "j" or "join"

P.S. VIDEO NOT WORKING? https://www.youtube.com/watch?v=sUGRsppR6HE&list=UUCmggVnEH_bpzaLnepgSUnw OR GO INTO THE UPDATE LOG!