Not sure if this is a bug or not but...
So, with the highest defense, not counting potions, you can apparently take a hit from a Dungeon Guardian and still survive (with about fifty hp left). Or, this has happened to me, while doing those oh-so-very-fun Dungeon Guardian runs (I, uh, used Red's Wings, somehow, but that's movement, not defense)...untold secret with Red's Wings, or bug?
All of tAPI's information is included within the thread, however if you're having difficulties there's a (rather old) tutorial I made at youtube...
https://youtu.be/QUI6ypHfSaw