How do I make sure I don't get attacked by Uranus?

On your first playthrough, Uranus isn't very aggressive. All enemies and bosses attack more frequently in Score Attack and Direct modes. And high-score boss tactics require keeping the bosses alive long enough to blow up everything they spawn first, so if you focus on attacking its polygons, Uranus will have enough time to cycle into its attacks.

In the first phase, Uranus will occasionally spawn missiles like any other enemy.

In the phases with purple polygons, the Uranus UFO will occasionally fire a laser below it while surrounded by its polygons and move towards you. You must damage the UFO specifically before it reaches you or the laser will hit you.