This is my mundo build when im dominating, kinda risky, but its worth it:
As is said before, this is for a risky, face smashing Mundo, max cleaver, then masochism, with a pint in agony in lvl 4 or 5. Also note this works better in TTL
If you want a safer build (like for SR) you can get Visage*FoN for better healing and ignoring mages, but this would set u out from the carry-ish build.
If you can farm like a god and are getting fed my ultimate build would be: Mercury,Visage,Mogs,FoN, Atmas,IE ... and if its one of those games that really go long you may get a Starks
As for runes Id go with CritChance reds, FlatHealth Yellows, Cdr or Mr/lvl blues