first off this guide to Mundo is not a guide that will tell you to build Mundo exactly or how to make Mundo invincible (i know about Warmogs abuse and the cons of it but it's boring). anyway have that settled let me tell you what kind of Mundo guide this is.

Mundo as a very adaptable champion, so building Mundo comes down to looking et your enemy and adapting your items to their base focus. (armor for ad's and magic defense for mages)

now for the practical things of this guide i will start with some basic strategy around Mundo (who to lane with who not to lane with) and then i will start focusing around the 3 f's settled in the title. feed,focus and fear

first of Mundo requires feed so Mundo goes solo lane.
sadly Mundo in solo lane is not always possible so here are some other options

Mundo with ap carry
Mundo with oftank
Mundo with nuke mage
Mundo with support (if you have 2 supports or have a support in 3 v 3 cause support goes with ad carry)

mundo can not lane with
ad carry.
anything like ad carry.
anything that needs to be fed and can't do it himself.

so assuming you are in a viable lane and you can concentrate on feed Mundo is really easy to be fed. because of his abilities which i will make sure to describe and work out in order

infected cleaver

Mundo's infected cleaver is a reliable damage dealer that has enough range to keep Mundo save from most enemies. using your cleaver on minions for farm is recommended but when used to harass an enemy one should keep in mind that the cleaver can't do more total damage to Mundo then it does to the enemy. (so really keep an eye out for that with any cleaver you throw at an enemy).

burning agony

mundo's burning agony can be used to farm in two basic ways.
first to just walk trough the minion wave and damage it till it's dead leaving yourself draining your own health and giving way to many minions away because you don't control your last hit's this early.
the second way of using it early game is like a free-to-low-cost basic attack by toggling it on and off very fast making low health minions die in an area around you for farm.

masochism

using Mundo's e early on is nice to kill a fairly low health minion but is not really that useful. the reason to get this ability early on is because the enemy might nuke you leaving you with low enough health for Mundo's e to be powerful and return the damage.

sadism

Mundo's r gives him the incredible sustain needed in a solo lane. make sure when you got it that you get enough damage over time in 50 seconds (and get the minion farm out of it) to make the ult worth it using to heal up and get back to farming

when you have your warmogs and tier one boots don't forget to get neutral mobs (low tier the other ones are for jungler and squishy humies) and minion waves to stack warmogs and get gold for other great items

so concluding in this chapter

q for farm
w for free basic attack
e daring last hitter
r epic sustain

oh and p.s.
watch out for nukes and assassins

playing Mundo as a tank is quite hard due his lack of taunts or other cc. this is why Mundo must focus high value targets and initiate in fights. if this fails the damage will be spread and Mundo ends up living and not being able to do **** cause he is a tank and tanks well yeah don´t do much but getting focused stupidly. but... Mundo does have some use but tanking things. Mundo with his e and Atma´s makes an awesome pusher that takes turrets down while not being able to die. and Mundo´s magic damage can help out the team fight with some major damage.

so concluding
Mundo must be focused
Mundo can deal epic magic damage
Mundo can break down turrets

pretty much says it all finalizing your build with some good items make you hard to kill and do major damage (sunfire, thornmail, stuff like that) while just not focusing on killing but on destroying high priority targets.

so now that you know how to play with mundo let's see how to build him

start with warmogs and tier one boots (tier two are unnecessary and with spirit visage boots can be soled completely)

i will divide the items into two categories (armor and magic resist) and specify how they can be useful most of these items will enhance Mundo's base abilities and stats which makes him even stronger then he is base.

armor
when the enemy has a fed ad carry or just has a lot of ad get this item right after warmogs and that should give you a lot bigger chance to win the game. this item works well as a pusher build with spirit visage and Atma's

mainly for pushing turrets with your e not very special although is nice combined with items that give some max health for obvious reasons.

really watch out with this one it is a lot of fun but makes it hard to push turrets when there are enemies nearby because turret focus will kill you if you don't have the necessary healing. good in combination with pusher build but quite tricky. but this one is good for team fights because there is no npc focus on you.
magic resist

my favorite item for enhancing your passive and ult, and giving your ult a lot less cooldown which makes it your base for attacking once in about 45 seconds. good for any mundo but is very good for pusher Mundo due it's enhancement of mundo's ult.

when enemy team is full ap this one is a good option against it also good when you have a hard time sustaining and have to recall after every but of battle you get in to.

so concluding on items

pusher mundo
other items adapting to choice or enemy team setup

teamfight mundo
yeah overall some damage from sunfire and your burning agony, good in teamfigthts and stuff.

pushing with ahri :) so much fun as ahri is hard to ignore or kill. wich gives mundo a free shot on the turret and when he does get hit it only benefits his e

starks fevor or will of the ancients
if someone has these and can get close to you hug him or her especially with will of the ancients mundo can drain some nice health making him a lot more annoying to come near to or kill.

[while playing mundo i advise you just have a lot of fun. when you haven't said Mundo while talking about yourself in the third person about 3 times a game, it sucks to play and makes you play bad. just have fun with mundo's stupidity and try (when fed) running laps over a 3v3 map. mundo is a fun character to play and has very much potential tanking different types of enemies.