Warning, this guide will be short and effective.
Build will work for normal and ranked games.




MUNDO

Good:

No mana
Spammable cleaver (and taking away 25% current health!)
Good CC
Loads of HP
Loads of HP regen
Fast

Bad:

No much AOE damage
Cleaver becomes damage wise useless if enemy is below 50% health
Masochism is not used much
Passive is kind of slack

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Early game:

*Grab a red stone and head to bottom lane. Best with a stunning lane partner.
*Chuck the cleaver every 5-8 seconds or so at the enemy because it does deal nice damage.
*Stay behind minions and use the brush.
*Don't attempt to gank because Mundo isn't great at that.


Mid game:

*By now you should have a good 5+ positive K/D/R ratio.
*You should have at least 2 complete items now.
*Use Sadism as much as you can. The CD should be 25% less of 60 seconds or so.
*Try and intimidate the enemy by chucking cleavers, showing off your resistance and by initiating team fights.

Late game:

*By now you should have a good 10-20 assists and even 5 or more kills.
*You should have most items now. You should be able to take on 2 or 3 players at a time.
*You should be initiating fights.
*Players shouldn't do more than 15% damage at any time.



ALTERNATIVE ITEMS:

I don't really like Warmogs and Randuins when you can have better items. 300 more health won't matter at endgame.

You can juggle the build around a bit, but just make sure you get more than 150 magic resist and more than 250 armour.

I hope you own.