When you make the substitute the shape of a Rhydon, can you really fool a Pokemon to fall for that. If you were using a Rhydon, then Mawile would have been fooled, but you're stretching my suspension of disbelief here.
Also, Puff used 40 HP to make the sub, which is the biggest you can make it, due to the usual DC, so if the Sub is broken in one round, then you've exceeded it. Focus Punch did 20, due to super effective damage.
And the combo was named Red-Hot Rhydon Plushie, so Puff breathed fire on his own Sub to make it red-hot, so it would damage it a little bit, but not so much due to not very effective damage.