How do you get Cresselia in Pokemon Rumble blast?

You can't get Cresselia in Pokemon Rumble Blast. It's not available in that game.

Cresselia is a Legendary Pokemon that appears in other Pokemon games like Pokemon Diamond, Pearl, and Platinum, and its availability depends on the specific game you are playing.

If you are looking for a specific Pokemon, you can check the Pokédex for that game to see which Pokemon are available and how to obtain them.