In order to get a perfect score you have to collect all gems in the demo there is 4 stages / screens in total.
Screen 1 - has 5 gems only tricky one to get is on the pillars at the end make sure to grab it on the lowest one before falling
Screen 2 - only has 2 the hardest being the big gem in the middle of the flying mobs, if you transform into them and hover to the top and down between them you can grab it.
screen 3 - only has 3 and all are gotten in the progress of beating this screen.
screen 4 (Final screen) - only has one and it is the hardest in the demo, if you line yourself up on the left of it and then move to the right of the screen while nudging up slightly you should fly straight grabbing it and passing through the spikes to the end.