This is the rushing build I use for Mundo. I find myself very hitting extremely hard late game with this build. This build is not meant to be played passively, if you are using it YOU MUST RUSH and be aggressive or else this build is no good.

All in all this build has one sole purpose: Annoy the hell out of the other team.

I always get cleaver first for ganking. Do not try to solo after the gank. Stay down bottom with a well synergistic team mate near by. Blitzcrank, Lux or Udyr usually works best. Anything with a stun e.g. Amumu will do though. With Mundo I usually go straight for an early game kill with a team mate who can stun. Always keep your distance but stay in range for cleavers. Your auto attack won't really help much but your cleavers will and they only have a 3 second cool down. If you find yourself not being targeted feel free to run in and stick to the target.

Depending on how well the early game goes, put pressure on the bottom lane. Keep them pushed back and keep them at 3/4 hp-1/4hp with your cleavers and your team mates help. Once the lane starts pushing back towards your tower push in for a kill with the help of summoner spells. (Exhaust and Ignite)

Keep pressing in for kills and your items should come in one after another.

Combat with Mundo:

Run in and connect with your cleaver first (If your cleaver misses, back up and re position yourself)

After connecting run in and pop Masochism and auto attack once or twice

Once the enemy is below 3/4 hp put up Burning Agony if you're still doing better than them HP wise

If your opponent runs pop Sadism and chase him with cleavers and burning agony.

Success!