I found out how to get a whole dungeon full of Prinnies, bottlemails
and shrooms.

Go to "Create a Dungeon." Read the enemy type. Keep cancling and
recreating until you get to a dungeon with the "weird" type. This
will be full of the rare monsters!

It usually takes about 50 tries or so, be CAREFUL not to cancel out
once you get the weird enemies dungeon, or you will have to start all
over! ( I made this mistake the first two times).

I haven't beaten more then a few floors of mine yet, I'm hoping that
when I kill enough bottlemails and Prinnies I will be able to create
them on the island.