The design in the video is not an exact recreation of the final boss, but it's a pretty good likeness, hence the giant spoiler tag. Even the YouTube thumbnail is a spoiler. So, if you have not yet finished Undertale, spoiler alert. Otherwise, hey, here's a cool Mario Maker reenactment of the final boss fight. Recorded by YouTuber garbotron, known as Corngamer on Miiverse, and posted last month (via r/undertale), the clip shows a Mario version of the Photoshop Flowey battle, with the two related tracks from the OST — Your Best Nightmare and Finale — edited directly into the video. The result is a surprisingly accurate recreation:
The music adds a lot to the Undertale feel, and the level design overall is really clever. Piranha Plant Flowey can't load save states, but he's as close to the real thing as it's ever gonna get.
The level's ID is 087B-0000-0155-27C0, in case you wanna try it yourself. Sans music, of course.