I don't think it would be all that hard...
if the plane is flying towards you. Watch planes on final approach. From the ground, it practically appears motionless. Aiming a laser at the cockpit shouldn't be difficult at all.
A shot from the side could be tricky, but since you don't have to lead the target, it'd still be easier than shooting a bullet at a plane. Plenty of planes have been hit by bullets (fortunately, it rarely happened outside of war).
The Air Force was working on laser eye protection for its pilots about 20 years ago. It really isn't that hard to dazzle a pilot's vision enough to make him miss his target or perhaps even to lose control.