Once you reach Deo, speak with him. He'll tell you the locations of the last two chests. After a bad pun, you'll receive the Jumbrella Cape. You can use it with 
to slow your descent as you gust around. You can also use it with Slipstream, similarly like the plants you've used earlier.
After that, head left to the next screen. Keep heading left, and bring a rock along with you when you pass one. When you reach the entrance to Wither Falls, you'll have to Vortex it open using the rock. When it's open, head left to the next screen.
Continue left over the fixed bridge to the next screen. Head up and left and use the same cave entrance as you did to enter the Slipstream dungeon earlier.
Right as you enter, Slipstream cape your way upwards through the small hole. Move left a bit, then bounce on the mushroom upwards and start cape-streaming your way up, following the right wall. When you reach the top, take out the enemies and head right to the next screen.
Keep heading right. Yeah, that's the chest already. That was a lot shorter than the path to the second chest, huh? Anyway, once you open it, exit the cave the way you came in and head back to Deo. Already.