Mission 15

You must destroy the Gate at the center of the level. However, a shield generator blocks off access. This generator is located towards the bottom right section of the level. Playing through the level normally you would likely take the platforms of the left and loop back around to get to the generator.

However, If you have trouble going up the platforms then try jumping constantly or climbing. At the top of the level is a pen full of fleas and when you try to take the platforms up they will jump on you and knock you off the platforms.