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Mundo has DPS if you buy attack speed items. Masochism is a ridiculous spell. +90 damage? How much would you love to have that on any champion? But in order to take advantage of if, you need attack speed.
Get Stark's. 45% IAS, 20% lifesteal
Get Malady. 50% IAS, 15% lifesteal
Get Last Whisper/Sword of the Divine.
If you stack IAS and get lifesteal, you can run in, pop Masochism, your ult, and wail on everyone while gaining huge amounts of life back. Go with Cleanse so that if you get CC'd you can get out of it and back to lifestealing ASAP. Use Burning Agony (lvl 1) to also reduce CC, and if you want, get Merc Treads to be completely immune.
The only other way I've played him successfully is to use Sunfire Capes to stack with Burning Agony, so you do 200+ dps all around you with tons of health, regen, and armor.