Dies to Removal

The shop owner claimed that Wizards said it works, but I have yet to find said "Wizard's Ruling". The way I read it; Monstrosity should be activated to become Monstrous (should read "has Monstrosity", but no one asked me in development) in order to receive said static abilities. +1+1 Counters are just icing on the cake and not the means to monstrosity.

So I a playing FNM Sealed and my opponent has Hundred Handed One. I have many flyers ready to swing lethal, and my opponent is short one mana to Monstrosity his dude. He tries to play a card I can't remember that puts a +1=1 counter on it and claims the reach ability is Activated due the counter activating Monstrosity. I counter it anyway and kill him. My question is on Monstrosity static abilities being activated by the +1+1 counter? I claim Bull shit as the ability says "when it become monstrous".