.::PurpleLife::. said:
Looks easy to do, but I've tried it before! It's so hard! >o<
I luv them :]
Looks like he just traced them in all honesty, not all that hard.
The line effects that look like brush strokes are easy to do in a single click in Illustrator, so all you would have to do is trace the lines of the pokemon with the pen tool or something, colour them and then as it's vector, you can enlarge them to the size that he has.