To do this glitch you will need to know how to do the duplicating glitch.

First get the light armour from the Arena. Then ready a match. Let them attack you until your armour breaks (make sure the game is on the easiest difficulty and that you have health potions).

After your armour has broken, duplicate it, then duplicate repair hammers (or buy them) and keep on repairing the armour.

If you're about to run out of the armour then duplicate some more. Keep doing this until you've got Mastery.