After playing against an Amumu main (he went tanky AP) I agree that damage is ridiculous.
We were winning early, but they were able to comeback thanks to him.
I'm honestly not sure how to counter him once he gets some core items.
Focusing him is kind of tricky in mid-late teamfights, and his ult isn't one of the easiest ones to bait.
Only thing I can think of is grabbing QSS (and its upgrade) after a core damage item or two, then baiting his ult that way and killing him. Problem after that is his teammates. He does a lot of damage at once when he ults, so burning everything on him could leave his team to ace you, especially if they have strong AoE like a Mordekaiser and Wukong combo.
Quite the dilemma.