This is the 2nd planet in the story mode.
They have replaced the "Press X buttons" with "Collect X keys".
The boss is Gigantic Bomber. He is so big he can crush you, and will chase you. Hopefully you've collected 8 speed power ups to make you super fast by this point.
His 2nd form is a huge mech that takes up most of the stage.
His attacks are a fire breath, and a "homing" Laser. You can avoid the breath by moving away from his head. When he fires the laser, a pink circle will surround you, that shows where he's targeting. You can just run away from this, and it'll miss.
To beat him, you need to knock out his four giant feet, and then his head.
Each foot needs 2 or 3 blasts. I suggest hitting his back feet first, as you don't get a lot of time to hit his head.