A simple and pretty straight forward intro.

So. This is a build for Dr. Mundo. It's fun. Probably. Depends on your skin colour. (purple)
SOOOO you want to know how to play Mundo huh? 'Cuz if that wasn't the reason you were searching for Mundo builds on google or mobafire or whatever, you have come to the wrong place. That's because I'm here to talk about a meh champion:
Dr. Mundo, the Madman of Zaun (or as I like to call him: the WTF Factor)

Moving on.

As you might have noticed, people don't play mundo that much. I don't know why. Probably cuz he reminds them of their Barney years. I can see how a broken champion like Tryndamere is played at every game (notice i said BROKEN not OP). You must absolutely use this as an advantage as other players are often unaware of your capabilities. This guide will show you the way, but not babysit your a**.

Summoner Spells.
Exhaust and Ghost.
Exhaust is pretty much a given cuz you lack hard cc's. go ahead and substitue ghost with other spells except for: heal(pansy), revive(******ed), smite(jungle? dumb****), clarity(troll), clairvoyance(somebody else will handle this).

Masteries.
pretty easy. 21/0/9. improved exhaust and ghost. Got offense because you wanna be aggressive with mundo. If you wanna argue with that, don't. cuz i might not care.

Skills.
Infected Cleaver = you being mundo. basically.
Burning Agony = aoe and cc reduction. thank you.
Masochism = Damge boost. Additional damage boost according to how many % health missing. This is the WTF factor.
Sadism = your ult. as you might have guessed, this changes you from the WTF how did you kill me, to the WTF how did you kill me and get away with it. It's heal is percentage based regen so more health is good. Its movement speed buff and heal percentage increases with it's level so you get it whenever you can.
I'll explain what to do with skills later.

Runes.
Armor penetration marks. It's good if you don't want an AD mundo. which you really don't.
Armor seals. notice what happens when you add armor to a tank. Quote: "WTF why aren't you dead"
Magic resist glyph. Again, same deal as armor.
I don't want to explain how it works but it does. besides you dont really need AD cuz of masochism.

Items.
start with ruby crystal cuz it is a recipe for spirit visage.
get boots personally would go for berserker greaves for the added AS but boots of mobility looks cool. Just remember that you NEED movement speed. will make difference between life and death. Or kills if you are overleveled.
sometimes i rush warmogs before visage. sometimes i don't. depends on the opponents item build. if they are a caster team or they just have a lot of magic damage, focus on spirit visage. If they are not, or they just suck as you got the first 4 kills, get warmogs.
again, if you need magic resist, and trust me, you do, get force of nature before atma's. Force also goes nicely with your passive and gives health regen. so win-win. otherwise, get atma's for added armor and DAMAGE BASED ON PERCENTAGE OF TOTAL HEALTH. woo hoo.
The last one is like a freebie. if you want, you can stack warmogs for more health and better atma's. Or you can go for stark's fervor if your teammates have not gotten it. it is great for team fights. Sunfire is good too, b/c it improves your burning agony as well. Frozen mallet is pretty much overkill. If you want to be scary and you have the gold, get madred's blood razors. ya. Wait, changed my mind. GET MADRED'S BLOOD RAZORS. period.

PLAY.
OMG that was too boring. I'd have apologized if i hadn't suffered more than you have when i WROTE this thing. so there, were through with all the technical stuff. Now on to some real strategies...

First let me list some rules. There are 5 in case you didn't count.

Rule number 1: BE AGGRESSIVE. Why? so you can make the enemy fear you. When people fear Mundo, they get killed, period. But don't overextend. Any good LoL player will know the difference between overextending and being aggressive. Plz plan a way out before pushing. Remember that creatures of the human race don't seem to like being made fun of by a giant, drooly, purpley... thingy. ya. ok. so use that for some more aggro idiots. Aggro against mundo is always good as it saves your team's carry and squishies.

Rule number 2: FARM. Mundo with a gold lead over the enemy champions is always fun. Don't forget to constantly check their b***hing when you rush warmog's at level 6. ha. ha ha. ha. You are a good farmer because of your masochism and cleaver. Practice hitting last hits with your cleaver and time your masochism so that you get last hit.

Rule number 3: SURVIVE. This rule is what people forget sometimes. I'm not promoting an invincible tank here. This is real life. THIS IS LOL. there are no 'invincible champions'. As mundo, you MUST survive. Also notice how I don't have Good Hands in my mastery? That's because you shouldn't die.

Rule number 4: TEAMWORK. DO NOT SOLO TEAMS(unless you're fed. if you solo ace on SR, kudos to you ******. stop braggin' about noob kills. :P but getting a solo ace on TT is not unheard of. i do it from time to time... notice how i'm bragging about my self without giving it away?). Don't listen to those a**holes that tell you to tank it for the team. DON'T. You are the mopper. You finish what the team starts, at the right time. A well-placed cleaver here and there can save allies. And get assists and hopefully kills. But really just 'ks' the **** out of people. ya. it's ok, you can be hated. but when you're fed, you'll win it for the team, ok? so it's fine. besides, beople who whine about 'ks' are people who don't have enought ballz or skillz to finish the job. GAY PRON

Rule number 5: CONTROL. You might think that as a tough/tank, all Mundo needs to do is spam auto attacks. WRONG. WRONG WRONG WRONG. Never stop moving. Always get into an advantageous position. Ex.: Bushes, to the sides where you have a clear shot, etc. Always get last hits on minions. Harass with your cleaver. It does GREAT damage as it dishes out percentage damage and also slows for a possible team kill. Always use your cleaver and masochism. In later levels use your burning agony in short bursts to blast through minion waves. Your skills cost almost no health compared to your max health and health regen you get in this build. Be the one in control. Pick your fights. Win them, or run when it gets too risky, but always stay in control. Mundo at the right place at the right time is DEVASTATING.

Now, your attack should be like this.
Infected Cleaver. Masochism. Hit. Infected Cleaver. Hit. Masochism. hit. Infected Cleaver. etc, etc. That's the basic.
Now add burning agony.
Infected cleaver. Burning agony. Masochism. Hit. Infected Cleaver. Hit. Masochism. so on and so forth.
Now add ult.
Infected cleaver. Burning agony. Masochism. Hit. Infected Cleaver. Sadism. (RAHHAHAAAHAHA where you goin' you slowpoke?) masochism. hit. kill.
These attacks are most effective against squishies and then tanks. DPS are a little bit trickier and it will make you run back and forth. like guerilla tactics i guess..

BUFFS.
Mundo likes his lizard buffs. the slow and damage over time is great. grab it when you have level 3-5 cleaver and have enough health. Maybe pop your ult if you're struggling. You should be able to get it pretty easily with cleaver and masochism.
Mundo likes his golem buffs too. with this your ult is pretty spammable lol. CDR on your already spammable cleaver is always fun and you'll ALWAYS have a 100 damage boost thanks to spamming masochism. Same tactic for killing lizard.
Mundo likes baron too. Now, what you need to do is hit the baron with your cleaver and run. You will get hit. It's ok. you can get hit. now you have to repeat this process until you are dead. Good. Now you have put the enemy team in disarray because they are not sure whether you are trolling or planning something else. you want this confusion and you should use this to your advantage. This is when you pop revive and clairvoyance for a suprise super quick gank. Trust me, this WORKS (maybe. depends on your friggin skin colour, b*****d.) Don't flame me because you're not purple, haters.
Remember when you are jungling where everyone is. being map-aware separates the meh mundos from the "OMG F***kin' feeder".

so yeah. i guess this is pretty pointless to say, but if you suck, you suck. Don't blame my build/guide. It's because you suck.

PRO TIP: when you reach level 6 and you are playing against seasoned players who know about Mundo's sadism, consider leveling infected cleaver. or masochism. depends on how much health you have. low heath = masochism, high health = cleaver. Knowing that most people level the ULT at lvl 6, you will have an advantage as they will back off thinking that you have your ult when you don't. this is when you have to be aggressive. The other advantage of having a lvl 3 cleaver or masochism pays off well here. They will try to run as they fear your heal and not really think about trying to kill you, so you can get a kill with the extra damage from your cleaver or masochism lvled up. hahaha. always fun to try this. TRY this. TRY. just try.

p.s. Do you want me to stop writing? Stop hating and go play LoL.

Oh, do you guys want some proof about how bad this build is?
k.. uh i got 1/23/1 once, that was really pro
then i think i got 3/16/5 so that was a little better
ya....