Dr. Mundo Build
Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Going for pushing early on using Burning Agonyto clear creeps (Unless you're getting harassed by ranged too much). This allows you to gain more gold for the minion kills and hopefully push the tower and then run off ganking other lanes and helping team mates. I find Burning Agony helps a lot more then masochism because at lower levels it can be used for pushing the lane and also doing a lot more damage to a hero at the early stages of the game.
Mid game you want to help slow any heroes you come across with your Infected Cleaver so the rest of the team can keep up. You may be able to take on some of the squishier heroes solo as long as you don't take too long for the rest of the team to come to your prey's aid. This is were your Sadism and Burning Agony comes into play nicely as you can put out some real good damage. If you happen to have Sunfire Cape at this point you'll be cleaning the plains with your fire of fury. (For added shock factor hide in a bush while on fire and jump out on passers by. This also applies in real life circumstances although is not advised as you are not as insane as Dr. Mundo and fire hurts).
If all is not going to plan try Angel Armour and make sure you're there with the team just to be a pain and annoy the enemy. Focus towers and clearing paths of minions. Your Masochism should bring any tower to rubble in seconds.
N.B. When attacking towers early on and you're safely able to attack tower for half a minute. Turn on Burning Agony and cast Masochism so you'll be doing more damage the lower your health. Just make sure those enemies don't sneak up on you ;]