Mewtwo's response to things that scare/disturb/annoy/anger him is pretty much to enter "nuke everything" mode. If It is harmed by belief, this is gonna be a problem, because Mewtwo's mindset is pretty much "I'm the strongest being in the world, nobody can hurt me, and I can defeat anyone under any condition".
This is sort of what I'm thinking of. If It turns into some of monster that Mewtwo is afraid of, I see Mewtwo's response as facing it head on and flinging Shadow Ball rather than turning and running away.
The asthma medicine worked like acid because the person believed that it was acid powerful enough to harm It, right? So if Mewtwo believes his Shadow Ball can hurt It, he would also be able hurt It, right?
How does It respond to fears of abstract things? Like if someone is afraid of death or loneliness. Does It have to transform into something physical?
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Pokemon VGC2011 NW Regional - Top 8
Pokemon VGC2011 US Nationals - 54th place