Firewalk with me
As outlined by GameXplain, there’s a glitch in Super Mario Maker that grants invincibility.
The process involves taking damage and walking through a door placed over spikes. The trick is in the timing — you can’t enter the door too early! — and the fact that you must go through it in Super Mario form (i.e., you need to start as, say, Caped Mario or Fire Mario and then get hurt).
If successful, Mario will be able to walk on spikes and right through most enemies. Ascension.
Should you come across player-made levels that seem genuinely impossible, this glitch might be why. I’d expect Nintendo to patch the bug, but that would put levels requiring its use in limbo.