6Hisoka Morow— he managed to not instantly die when Chrollo's Sun & Moon detonated, meaning that he can take a serious beating. Of course, Hisoka has enough power to handle Meruem, and where he lacks aura, he resorts to trickery, strategy, and the benefit of unpredictability. Can killua...
Hisoka doesn't have the firepower to beat Meruem. Chrollo doesn't as well, although considering his ability he can use Sphinx's Cyclotron and some of his friends abilities to aid him, and he will need to set up an environment where he can win. However, in a straight up 1v1 n...