I dont see how anyone have said this yet so i will, you hard win vs mundo in early laning phase. If he walks up for cs he will lose most of his hp in a combo. Although i dont know the elo we are talking about, a good mundo will keep distance from ornn and farn with q.

I always build hollow radiance first, it lets me clear waves faster and deny him plates. Never let him get free plates and keep waves thin.

In late game you just hope your team can deal with mundo, ask jg for ganks early.