How to beat King Boo in phase 1At the start of the fight, there is only a single version of King Boo, and what you need to do is avoiding his fireball and tongue attacks until the point where he throws a spiky ball towards you. When this hits the ground, it will let out a few bombs, and these can be picked up by your Poltergust. Now you must quickly aim this at King Boos mouth before the bomb explore. It can be a bit tricky to get the timing right, but you will eventually get the hang of it.
If you manage to get the bomb into King Boos mouth, he will be stunned. This will allow you to get up to his tongue and pull it. You need to use both Luigi and Gooigi to manage to throw him and cause damage to him.