Don't stand near the edges of the screen. It's dangerous.
Sketch shoots in a specific direction depending on the page: if left page he shoots towards the right and if right page he shoots towards the left.
As in most games pick up gold coins whenever possible. You'll need this virtual currency to buy weapons and items in the Shop.
A bomb will not only kill aliens, but also blow up other bombs in the area.
You can activate a preview of each weapon before making a purchase. And these new weapons you can purchase.
Use an even greater edge on the battlefield and purchase upgrade: utilities, bombs, helth etc.
Sidekicks and undead upgrades should help you even more in your task overcoming the obsticles.