Mundo is a hard champion to kill, and you have to play with that.
Its good to max the first skill A.S.A.P. for the harass. Mundo is very good aggainst HP tanks like Mordekaiser or Vladimir.
The third skill begins to be very important when you have 3.2k or more HP, because it gives flat damage plus damage for percentage of health missing.
Save your Sadism skill (ultimate) for when you want to tower dive or chase your enemies, or when you are being ganked, because if you are low in HP, you can always take part from your PASSIVE skill. Just lay low in yout tower and wait for your HP to regen.
When being ganked, spam your first skill back and keep your second on if you have more than half of your HP bar. It helps you aggainst stuns, slows and snares.
Well i think that is all, if you need to take any extra doubts, just PM here or in-game, nick kimagure :)