this is going to be unlocked through natural progression of the missions, if i could give any advice on how to speed this process up it would be that you tackle everywhere on your map which you can access by hitting
and seeing your map, it does show you when you've entered the area just go to all the outskirts and avoid the main objective until all mobs are defeated! complete this on longer missions like Redstone Mines and a secret mission that you can find in the mission Creeper Woods i do suggest looking at a guide to find that bonus mission as its very helpful for leveling up and mob farming! it's called Creepy Crypts, run through those a few times and you should get it in no time!