First of all, sorry for not catching the fight closer. The Mok'Nathal commander is the the one striking Cho'Gall with the big sword.
But, incoming spoilers and pics:
The quests before are not that important, catching a orb from a Gordunni, capturing a Gordok with a Soul Stone. We find that Cho'Gall is in Dire Maul recruit Ogres back to the Twilight Hammer. We go the Dire Maul Arena and we find this happening, Cho'Gall giving a speach to the Ogres and sending us the disciples.
After we kill the disciples, he intervenes, freezes us and begins casting Flaming Pillar of Certain Doom. That's when Orhan Ogreblade and the Stonemaul appears. She is a pure badass, Cho'Gall was around 640k hp and we see his life rapidly lowering after Orhan's attacks (including a might attack). Cho'Gall runs from the battle.