Let's face it, Mundo can be pretty ignorable built as a tank. So, let's give him some more damage. With Mundo's E, masochism, between 100 and 200 AD is ADDED to his base damage. That is CRAZY. So, we can use this as an excuse to not get AD items like Bloodthirster, and be able to stack those fun items, Phantom dancers.

Pros

-Great mid-late game
-Very fast
-High Crits
-Can solo Dragon and Baron (late)
-Surprising

Cons

-People think you're trolling
-Not tanky, not much use out of his Ultimate
-Vulnerable to CC

Critical strike chance runes for reds and quints, your choice for yellows and blues. Usually armor and CDR.

You want to start with Brawler's Gloves, or Doran's shield (if you're solo top). Build this into a zeal. Berserker's grieves are the best choice for boots. Start building one or two phantom dancers. Build infinity edge when you deem apropriate. If you find yourself dying a lot, Guardian angel is a good idea.

Since this build relies heavily on crit chance, make sure you end up with around 80% or more.

Level masochism along with your cleaver. Sadism at 6,11,16.