Kologarn himself has 21 million HP. and his two hands are seperate mobs having 5 million HP each.
Kologarn is located at a very cool place that he acts like an obstacle between 2 platforms ,so after you kill him he becomes a walkway, resulting you guys access to other bosses located behind him, so no way he is optional boss,you raid must kill him in order to access many other bosses like Auriaya, General Vezax,all the Watchers, and finally Yogg-Saron.i find it nice and new as the bosses are getting more interactive with the architecture of the dungeons.