Oh damn it. Sorry, im not too good with the legion... so much for thatWhat... first off Illidan needed three others(From what I gather they were Kael'thas, Lady Vashj, and Akama. Not exactly weaklings assisting him) to beat Magtheridon. And Magtheridon was Mannoroth's underling, do not confuse him with Mannoroth. Mannoroth was the king of pitlords, the strongest one there was.
Second, Mannoroth didn't kill Grom. Pitlords of power like Magtheridon and Mannoroth would explode in hellfire when they die. Mannoroth didn't do any kamikaze attack, Grom's axe destroyed his heart triggering his body to blow up. Grom himself triggered the explosion not Mannoroth.
Mannoroth = Kil'jaeden's second in command
Magtheridon= Underling of Mannoroth that had thoughts that maybe the siege on him was the Legion testing him. Doesn't sound like a guy near Mannoroth's level