Some supporting art for Illusion.
Original Design: http://i800.photobucket.com/albums/y...APShaman-2.jpg
Shaman moth mon wraps itself in the cosmic power of its wings to manipulate light and form false images of other pokemon. Here we see Shaman mon about to disguise itself as a tyranitar to lure a fighting type into a sense of false security
(I know tyranitar is a bad example because of sand stream, but please bear with me).
Also on other abilities, weak armor can work similarly (moth wings aren't very good armor) and as far as no guard (which is looking like it will win the tertiary ability poll), shaman mon is always praying. It would rather pray and take an attack than worry about fending said attack off.
I must admit tho, as far as logic goes, having an ability like no guard while also having an ability that involves armor is a bit hard to have justified at the same time.