Mundo is, in my opinion, a really funny champion to play.

Because mundo's skills cost HP naturally needs to be tanky.

Please remember this before hate me:

1.-Im not a pro player
2.-mundo is not my main champion
3.-because i dont play a lot, my runes and masteries are not the same i use (i mix the runes and im at the moment level 28)

Pros:

*EXTREMELY tanky when reaching level 6
*no need of AD items due Blunt Force Trauma
* Infected Bonesaw is really useful to slow enemies

Cons:

*your ganks will fal if you miss Infected Bonesaw
* Maximum Dosage can be countered easily by Ignite
*you may struggle if under fed

this runes are just the common top/jungler runes, almost any champion in these positions use this runes, and they work good for me

if you go jungle, you will probably need Heal in early game, because you may spam skills to clear your camps fast, but don't worry, Heal is really not necessary, if you think is the best option, don't be afraid to get Flash or Ghost

IMPORTANT NOTE:

this items are what I usually use, try to take the best of this build and change anything if you need it.

if you go top mundo, you will need to max out Q first, this is to poke the enemy laner and slow them in case of ganks...

But if you go jungle mundo you may max out the E first, because in the jungle you will (at least in early game) get harmed really bad, and you can use that to your favour...

mundo' W must be used with extreme caution, because if you use it for a lot o time, you may lose the half of your heath just by having it active, if you want to use it frequently, you should get a spell vamp object...

Talking about mundo's ultimate, this skill can save your life or gives ou a faster death, be careful when you use it, because it cost health but at the same time, regenerate it.

in the jungle you start in red buff, because you do not use mana, and you may try to give the second blue to your mid laner...

as i said, you start at red camp, then go for wraiths and wolves, finally go for the blue buff and try to gank at level 3...

honestly i do not recomend counter jungle because yo dont know if the enemy has warded its jungle, but if you want to try it just remember to lave a minor creep alive to make the enemy creeps respawn later...

if you go mundo top, try to farm behind enemy minions, and trow your Q to the enemy laner, this is to prevent that he farm, just remember to ward the bush in river and the one en the entrace to your jungle to prevent ganks...