If the spell you cast with the cascade ability is a creature, it will have entered the battlefield this turn when Let the Galaxy Burn resolves and will not take damage from Let the Galaxy Burn's effect.

If Knight Rampager has multiple attack requirements applied to it, usually because it was also goaded, you must obey as many of the requirements as possible without violating any restrictions (such as effects that say it "can't attack") that might apply. For example, if the player chosen randomly also goaded Knight Rampager, then Knight Rampager must attack, but would be allowed to attack any player.