Actually this is very easy to get.
First, Make a NEW HERO and pick the BARBARIAN.
Enter a QUICK BATTLE and scroll through (LB or RB) the monster until you get GOBLIN and battle it.
The only two attacks the goblin will do are SNEAK ATTACK (3 damage) and SHORTSWORD (3 damage). As you battle the goblin its very important that you don't let it get any 4 or 5 matching skulls or any +5 skulls. This is because the Barbarian's starting health is 34, so if the goblin kept doing attacks of 3 you'll end up with 1 hp if you let it.
So fight the goblin till you get his hp pretty low, then just start setting up skulls for him but to avoid letting him get any 4 or 5 in a rows or +5 skulls. When you hit 1 hp, just use Pummel or match a skull and there ya go!
Edit: If you get a 12x12 board instead of the 8x8, it might be a good idea to exit and start another battle because it's difficult to control a 12x12 board compared to a 8x8.
Also It might help to put it on Easy, the goblin's health doesn't change, but I noticed that he sets up less chains on Easy.
And don't forget to not let those +5 skull fall near other skulls, let the goblin do that so you can use them.