Brox has proven himself more skillful than Grom at numerous occasions. Of course, Brox only inflicted a cut on Sargeras- whilst Grom slayed both Cenarius and Mannoroth. I just say what I always say- there was a lot of luck involved.
Brox piled up countless numbers when he walked trough the Burning Legion portal. He litteraly piled up all whom attacked him until he stumbled over Sargeras and was defeated utterly and humiliatingly. Grom on the other hand, in the earlier years, had some difficulties defeating average Draenei in a heads-up battle. Of course, the Draenei is highly skillful warriors, but considering the things Brox has defeated- Brox has proven more skillful. He fought three Doomguards by himself and lived.
Grom defeated Cenarius, a man who proved himself powerful enough to have crushed Grom with a motion. It was only luck for Grom to even reach the forest God. Then he defeated Mannoroth, who in truth wouldnt even need a motion to defeat him. A simple incinerating glare should have been enough.
While Brox has done what he did efficiently, there is a lot of luck revolving the story of Grom
