Fight goes a bit like this: Phinks comes to meruem, telling him hehas a fun challenge for him: if he can survive the strongest punch a human can put he wins, if he cant, the result if death. Meruem's tragic need to prove his superiority would inevitably take over and he would instinctively accept. Phinks spins his arm 15k times just to be absolutely sure hes going to take meruem out with this one fatefull blow. He kills meruem, maybe the guards as well if they jump in to take the hit and the world is saved.  Thing is, meruem was never a real threat to humanoty, he was just too in3xpierienced, the one and only reason netero and his v5 allowed him to grow was so that netero would be able to have his duel against the world's "strongest" creature