For Masteries I take 21 9 0, taking all cooldown reduction, Armor penetration and Magic penetration in offense. And in the defense tree i take 3 in both Armor and Magic resist, and then 2 points in HP per Level, lastly I take Veteran's scars for more HP early game.
I don't go in to the utility tree as Dr. Mundo doesn't have mana nor energy, all his abilities cost HP.
For items I start out with an Doran's Shield as it grants you both Armor, HP and HP regeneration, which you need.
When playing Dr. Mundo you should make sure that you will be alone on a lane, preferably solo top. Dr. Mundo has the advantage of not using any mana or energy for his abilities, making him only having to go B when he is to low on HP or having to much gold...
When you go B the first time you should start on getting Spirit Visage by buying Kindle gem, or if you didnt leave the lane untill 15min or so you should buy Spirit Visage and Boots of Speed. When you have enough money you should start on Warmog's Armor, start of by buying Giant's Belt and then work your way to finnishing it...
After you finnished either Force of Nature or Sunfire Cape you should get Atma's Impaler which gives you both armor and crit chance. Armor will help you surviving and the crit chance you get will help you do more damage, and it will also give you alot of more AD, since it scales with how much your max HP is, so it gives you alot of AD
After you got your Warmog's Armor you need to think; do I need more Armor or do I need more Magic resist? If you feel that you need to get more armor then you skip getting Force of Nature and get Sunfire Cape right away, but if you feel that you need more Magic resist then Armor you get Force of Nature instead of Sunfire Cape.
If you want you can get another Warmog's Armor instead of Sunfire Cape but I recommend getting it instead of another Warmog's Armor.
Here is an description of all his abilities and his passive:
Adrenaline Rush Dr. Mundo is granted 0.3% of his maximum health as bonus health regeneration per second.
This means that the more HP you have the more you regenerate, wich makes Warmog's Armor an great item on Dr. Mundo.
Infected Cleaver Dr. Mundo hurls his cleaver in a line to damage the first enemy it hits, dealing magic damage equal to a portion of the enemy's current health and slowing them by 40% for 2 seconds, it has a minimum damage threshold. If Infected Cleaver hits a target, it refunds 50% of the cost. It has a maximun damage cap versus monsters and minions.
This is his Q ability and it is one of the greatest chasing abilities, as it does alot of damage and slows the one you hit with 40%
Burning Agony While active, Dr. Mundo will deal continuous magic damage per second to nearby enemies and reduces the duration of crowd control effects used on Dr. Mundo.
This is an ability you should only use when you got your ultimate before you got your Warmog's Armor since it will make you loose alot of HP very quickly on low levels.
Masochism Increases Dr. Mundo's physical damage by a flat amount for 5 seconds. In addition, Dr. Mundo gains additional damage for each percent of health he is missing for the duration.
This is ability gives you alot of damage boost for 5 seconds, and it will give you even more the lower HP you got, if you have lost 90% of your HP you will get another 90AD, wich means if you are on level 18, and you have lost 90& of your HP you will get a total of 190AD plus the AD you get from the extra AD from Atma's Impaler
Sadism Dr. Mundo regenerates a portion of his maximum health over 12 seconds. Additionally, he gains extra movement speed for the duration.
This is the signature of Dr. Mundo as it will bait people to attack you since you loose 20% on activation, but they will be the one dying, since you get so much HP regenration and the AD, if you have your items you will be a threat, as you will be able to take a ton of damage and still survive. And your ultimate will be on a 58,6 sec cooldown if you keep your Spirit Visage
Now my friends, you have made it to the end, and I would like you to vote and comment on the guide, please tell me if you find any grammatical errors, as english isn't my native language...
I hope that you liked the guide and are willing to share it to your firends
I'll try and add more information as I find more items to add...