Very, very hard. Worst is you want all the achievements. Rage quit is not an option, it is the solution. When games are too hard, it is not a pleasure to play them. I don't recommend it for this reason.
I love this game but it's currently impossible to get all of the acheivements due to a glitch in the bonus stages of two levels in world 3. I have filmed these glitches and sent them to the creators and recieved an email saying, "This is really helpful thanks." But there hasn't been any update to the game and I don't expect there ever will be. It's a shame, too, because I love this game and really want to totally complete it.