if you aim well, you'll get much more points for the same amount of killing. For example, when using 'fireball', you will shoot 3 fireballs at once. the enemy killed by the central fireball will give you at least twice as much points than if it's killed by the other balls. I'm level 117 now. Sometimes i get up to 185 for killing an enemy with the central fireball, otherwise i get around 65 points for the same enemy. so i guess aiming well is better then going around killing a lot of enemies..
there seems to be some kind of bug related to double jumping. Sometimes, when you jump, it makes the 'sound' of a second jump, even though it is the first, and you are unable to do the second jump. I tried to figure out if there were specific situations for this problem... the only clue i have is, that it seems to be related to "pointy corners", like the ones on the inferior part of the smallest pieces of land. If you are below one of these and trying to make some maneuver with double jump around it, it may happen this problem, and you are unable to perform the second jump.
if you're having a hard time killing the boss, just go around killing some beasts to increase your power (level up), as your shots will be stronger after this, it will be easier to defeat the boss
in room 4 there´s a piece of music hidden on the floor, you have to turn on the lamp and move the picture, so that it will appear under the bench. Is this what you were missing?