Im going to make this short and sweet, Mundo is probably the best tank in the game other than singed. The key to playing mundo successful is by having a ton of health and armor and magic resist. I may only be platinum but I have so many games as Mundo.

Mundo is super tanky early game I prefer burning agony for that early game team fight. Always rush sunfire cape, combined with burning agony it does a lot of damage. Burning agony and sunfire cape are the huge portion of his damaging abilities. This is the reason why you must have at least some form of magic penetration. Magic pen does work with sunfire cape and his cleaver as well. Masochism is a good key for standing fights as well but his aoe damage is where it is at. I dont recommend the jungle items because mundo will have so much health regen late game so there is no need for all of that. You dont want ap on mundo just magic pen. That is the key. AP only works and scales poorly on burning agony.