I play one puzzle a day, I haven't bought any picrites at all, just saving up from daily training, achievements and missions. I've also unlocked the Mega Pencil this way (that took me months) and now I'm on world 20-something, not sure exactly.
Ther mega pencil enables you to do the mega evolution levels. They give you a few extra picrites (missions and achievments) and the pokémon are basically the same, but more powerful versions of their non-mega evolved forms. For example, Mega Pidgeot is a 20x, while normal Pidgeot is a 15xGlad to here there is someone else chipping away at it without paying anything.
I forget what does the Mega Pencil do? Well I know it unlocks certain levels but do those pokemon you get in those levels help?