Why is DF such an insane beast? He clearly a level above Luffy in this arc...which is a first for OP. Luffy always managed to elevate himself to his limit to take out the boss, but this time DF has had many fights himself and many handicaps. Added to that, he has taken many blows from Luffy, much more damage from him and Law and come out of it...Pretty crazy.
Well he basically runs the criminal underground and most of the black market activity in the new world, right?
He's involved in the the slave trade, devil fruit trafficking, even development of WMDs apparently, and is the main supplier of Kaido's forces. Possibly also the source of the bomb that Crocodile had.
He's also one of the warlords and has at least two type of Haki (have any villains had more than one so far?), and has had mastery over them for a long, LONG time.