if you have ever heard of the lol saying mundo goes where he pleases? Thats true he has insane health speed armor and magic resist. He also packs a punch with large amounts of attack damage and attack speed. So as you can see Mundo is very tough to beat WITHOUT any of his ability's. Mundo is like Olaf were he becomes much stronger the lower on health he is. this is mainly because of his ultimate and his E both of which benefit him more the lower on health he is.

Tanky uses health instead of mana
High M resist doesnt have resist early
High Armor item dependant early game
high attack attack only gets really good when low on health
no mana
low health costs

you want to start a fight by a little harasing with youre cleavers. Then follow up while there slowed with a charge into the enemy (early game stay away from towers) activate youre burning agony and throw youre cleavers to keep them next to you finally activate youre E to increase youre damage at leas do this after using youre E and youre first use of youre Q so that you get the bonus ttack damage from health missing.

mundo is one of the best people in the game (like teemo) at getting out of fights. but unlike teemo he is also very good at running into their entire team and surviving intense beating. if they have people who are slowing you keep youre W on this will reduce the time of the slow.

Mundo relies on his tankyness so dont play agressive until you have a giants belt then go for kills more. Mundo is one of the tankiest people in the game but you need youre items so be wiling to towerdive mid game.