Guide Bubble Girl (Wendy) trough 40 levels.
This is done by defeating all enemies on stage.
It is very simple doing so, as you need to stun them with bubbles and when they get stunned, push them out of the play area by letting Wendy to touch them!
Each enemy attack a bit different, some can jump, some can shoot, so in case that Wendy got hurt, find a heart to replenish her lost health as well as some power-ups like triple shots.