Who beat Meruem?

There is the possibility that Adult Gon possesses enough aura to defeat him, though (at least, according to Neferpitou’s fearful rambling). As of current, there is absolutely no possibility that anyone can defeat Meruem in a one-on-one fight.

Did Meruem get defeated?

Meruem is assaulted by a barrage of palm strikes that burrow deeper and deeper into the ground until the two fighters break through the ceiling of a massive grave. Netero strikes Meruem again, sending him crashing into a pillar.