Magic Pen Mundo Howling Abyss
Dr. Mundo Build
Magic Pen Mundo is a good alternative to regular Mundo if you need AP damag
Start pen boots. Liandry's should come next, unless they have a lot of magic resist at which point you could build the Haunting Guise and get a Void Staff before finishing Liandry's. After Liandry's and Void Staff you can build more damage or go full tank depending on the enemy's team comp.