Thanks Xenocidius for your explanation. I think that the code is like you described.
However, if Zanz coded it differently, the solution is to make the passive skill Mimic handle the 0 HP status only at the end of turn.
@OldTrees: i know this cards have many vulnerabilities, and some of these are quite serious (Chimera, for example). I thought that these vulnerabilities would balance its "instant Fractal" effect, which is really powerful.
You're right about the ability to play skill the same turn the ability is gained. Exactly like when you mutate a creature that it wasn't played that turn.
I was thinking that another base form that Shapeshifter could take was a creature with -10|1. This way the creature would be less vulnerable and you're not forced to mutate it right away (but you're punished for not mutating it). That's was just an idea, though.