Grab the Bee Mushroom
The first thing you’ll need to do is head up to the higher level at the top of the hill on the starter planet. You can do this by heading around the back of the cliff and wall jumping up to the top. Once you’ve become Bee Mario, fly over to the flowers that are going across to the large redwood tree area. Watch out for the rain clouds though! If you’re caught under one of those then you’ll lose Bee form. You can easily just fly over or around them. Bee Mario can actually use the clouds as platforms since he is a lot lighter than normal Mario.
Wooden Platforms and Mandibugs
When you’ve made your way across the flowers, you’ll come to a wooden platform with a swing. Use the swing to give you a boost into the air and then fly onto the clouds. Fly hop from one cloud to the next until you get to the wooden hexagon platform. From there, make your way across the wooden structures. Make sure you avoid the stormy clouds and the Mandibugs. You can fly over the Mandibugs or groundpound them to defeat them.
When you’ve made your way to the end of the wooden structures, use the spring jump to take you up to a swing. You can then fly up off the swing to the observatory tower.
To get to the top of the tower, you’ll need to make your way up. Along the way you’ll need to make a series of stops up the sides of the tower. There are a number of rain clouds along your path, so you’ll need to hide from them or move out of their way otherwise you’ll lose your Bee form and need to start again. On the highest platform you’ll find a concrete stump that you can groundpound to reveal a Sproutle Vine that’ll take you to the top of the observatory.
Undergrunt Gunner Boss
When you reach the top of the observatory tower, you’ll have to defeat the Undergrunt Gunner. He is surrounded by a spikes, with moving platforms that go from one end of the observatory roof to the other.
Bee Mario - The Gunner will be shooting bubbles in your direction and remember, if you get hit by a bubble, you’ll lose your Bee form. Fly over to the Gunner and groundpound the glass above the undergrunts head.
Normal Mario - If you are no longer Bee Mario, then you’ll need to use the button at the back of the observatory to help you defeat the boss. When you groundpound the button at the back of the observatory some platforms will raise near where you started. Quickly head back over and use the platforms to jump up onto the Gunner’s machine.
You’ll need to groundpound the glass of the gunner’s machine three times to defeat him for good. Each time you hit him, he’ll get angrier. During the third phase, he’ll be extra angry and he’ll shoot his bubbles at you even faster, making it trickier to avoid them.
A Star will spawn once the Undergrunt Gunner is defeated.