Infected Cleaver is very effective in taking out neutral camps. It deals damage base on monster's HP, so Dr. Mundo can jungle effectively.
Because Infected Cleaver makes a distinctive splat noise and returns health when it hits an enemy, it can be used reliably for checking bushes for enemies.
Since Burning Agony also gives you crowd control reduction in addition to the damage, don't forget to use it during team fights or while chasing/fleeing especially against crowd control heavy enemies. Also, the reduction does stack with Mercury's Treads to make crowd control very ineffective against Dr. Mundo.
A well-timed Sadism can bait enemy champions into attacking you even when they lack the damage to finish you. Force enemies to chase you across the map in their attempt to finish you off, if you can- sadism's speed boost combined with the cc reduction of Burning Agony makes taking out Mundo frustratingly difficult.
After you reach level 6, you should rarely have to leave your lane. Although Dr. Mundo abilities use health, his Sadism has a fairly short cooldown so you can regain lost health about once every minute.
Sadism continues to heal you when you're resurrecting from Guardian Angel. Timing a Sadism well with Guardian Angel is a very good way to bait and finish off low health opponents who wait for you to resurrect.
Due to a tendency for many players to counteract Dr. Mundo's health regeneration with Ignite, taking Cleanse as a summoner spell might seem like a good choice; however, it does not actually remove the healing debuff, only the DoT. It does combine well with Burning Agony to cause strong crowd control resistance, though.
Since Sadism is a regeneration buff and not a heal it will give you the most survivability if activated right before you take major damage. Keep in mind the regeneration speed relies upon a percentage of your maximum health, so stack up for the fastest regeneration possible!
Spirit Visage is a very effective item for Mundo due to its 20% boost to regenerative effects, cooldown reduction, and extra tankiness, all of which benefit Dr. Mundo
Spirit of the Ancient Golem is good too since it gives you a bunch of HP, faster clearing and more Tenacity.
Warmog's Armor offer a lot of health regeneration without any life steal, even more when Adrenaline Rush and Sadism are taken into consideration. With it, Mundo's health regen will outpace the health cost of Burning Agony.
However, this counter loses its effectiveness if you build magic resistance. Mundo with Mercury's Treads and Spirit Visage will have around 120 magic resistance.
Stacking health and getting an Atma's Impaler can greatly boost your damage, synergizing with Masochism, as well as increasing your survivability.
Sunfire Cape paired with Wit's End offer Dr. Mundo a lot of valuable stats - health, armor, magic resist and attack speed.
Magic Penetration is overall more useful to Dr. Mundo than ability power, because Dr. Mundo'sInfected Cleaver gains no benefit from ability power since its damage is based on enemy health, but still deals magic damage. Sorcerer's Shoesor Haunting Guiseare all superb choices for anyone wanting to increase the damage output of Mundo's Cleaver and Burning Agony.
Debuffs from Executioner's Calling, Morellonomicon and Ignite can be an extremely effective counter to Sadism and weaken his regeneration tremendously. If playing against a good Dr. Mundo one of these three should always be available in team battles.