Can Harvey Walters solve the final rune puzzle with his special ability?
There is nowhere mentionning you cannot use the character's power against the "boss"

I would find it rather hard to believe you can use the statuette that reduces the boss's damage to harvey to 1 but couldnt save the 1shot power versus the "boss puzzle"

Anti-climactic yes, but then again what's the point of wasting a good power to get a 1shot use if you cannot use it when it really counts