Not sure if you know the easiest way to build up... read on if you want the cheat a little.
When you enter some dungeons there will be the statues right in front of the palace and if you hit them either a red potion or one of those armor guys will pop out. Do it so you get an armor guy, kill him, leave to the overworld screen, re-enter the palace and do it again. There is no limit as to how many times you can do this, a random item will always re-appear in the statue when you re-enter. You just cannot beat the palace first because it turns to stone once you are done and you cannot go back in.
I hope that makes sense.