Okay, for this you need a human who can cast magic and a room with a monster or two. You also need the spell Ryuker and the difficulty level of Hard, Very Hard or Ultimate.

Once you've aquired these prequisites, find a monster that can turn into a rare monster:

Rag Rappy: Al Rappy
El Rappy(Ult): Pal Rappy
Hildebear: Hildeblue
Hildelt(Ult): Hildetor
Blue Slime: Red Slime

When you encounter any of the creatures to the left, use Ryuker and teleport back to Pioneer 2. Go back and see if there's a change in appearance. If there is, you found a rare monster. Kill it for a rare weapon or Armor. IF not, try again. Some monsters change from Very Har to Ultimate, so (Ult) means they can only be seen on Ultimate.