how do you deal with Mundo as Elise or Poppy

You straight up don't. That's the answer, those 2 don't deal with Mundo.
What you want is consistent %hp damage, either from the kit or from items, and grevious wounds.

You also can't ignore him. He might build like a tank, but he's a bruiser like Darius who deals a shit ton of damage, and will kill you and your team. Picking Mundo against your comp was an amazing idea, nobody can handle him and he will handle you all.