This the final level in which you will need to save Luigi. You will need to have already done Luigi and the Haunted Mansion, Luigi on the Roof and Luigi under the Saucer (in order). This level will become available once Mailtoad has given you a letter indicating that Luigi is in Honeyhive Galaxy.
Become Bee Mario
First things first, head up to the fountain at the top of the hill and pick up the Golden ? Coin. This will make a Bee Mushroom spawn below, near the waterfall. Head down and grab that mushroom to transform into Bee Mario. Now, you’ll need to fly up to the waterfall level, but watch out you don’t touch the water as you’ll lose your bee powers.
Next, head to the edge of the platform, where the flower bud is. From here, you’ll need to hover and fly over to the edge of the otherside of the planet, which is to the left of the waterfall. You should only just about have enough flight to reach the edge, but if you land on the fence for a brief moment, you’ll gather enough extra flight power to avoid the pond and fly safely to the grassy area without losing Bee form. It isn’t vital to keep Bee form but it’s handy.
If you’ve kept Bee form, head over to the wooden wall that surrounds the outskirt of this grassy area and fly onto it. If you’re not Bee Mario, you’ll need to walk around the tree a little to get to a lower section of the wall that you can jump up onto. Make your way to the top and then head past the pen and to the ledge that overlooks the waterfall. Here, you’ll find Luigi clinging on for dear life to a tall tree. You can either fly up and jump on his head as Bee Mario or you can shoot a Star Bit at him to make him fall. Once he’s fallen, Luigi will give you a Power Star!