After you get the border, ears, cheeks, and part of the eyes it gets harder. If it seems like there's nowhere to proceed, you probably made a mistake.

No mistake, entirely solvable.

Not logical....but the picture is very very beautiful!!!!!
Hint: perfect simmetry from low line to the 20h (except the 13h... the inside eyes)
also the high four lines are simmetrical.... enjoy smiling pikachu :-)

Let me clarify my statement. When I said "you probably made a mistake" I was talking about the puzzle solver. The puzzle is solvable (you might have to use temps if you can't see the second order logic), as long as you don't make any mistakes. Some puzzles are forgiving of mistakes -- you can still complete them and then fix up the mistakes after the fact. This one is not. If you make mistakes you will get stuck.

I found it completely easy.

I want you!!!