As usual, the bosses in the main story quest have a pitifully low amount of HP where they died so quickly before I could register what happened, and I am playing on Inferno 4. 
Well, unlike previous end bosses in every main story quest, at least Diablo was treated better where he has decent HP not to die immediately, and actually puts up a decent fight. 
I still think that the current bosses in the main story quests should have at least a 100% HP increment and a 50% damage increment to provide a decent challenge.