Does Dr. Mundo's Infected Cleaver ability reduce monsters' health by 25%? If it did then I can see Mundo soloing Baron and dragon at level 14 or so. Paired with Soraka, he could do it at level 11.
Dr. Mundo's cleaver does a maximum amount of damage versus neutral monsters, increasing per level.
The values are:
Remember that this is based on current health, which means the second cleaver will always do less than first (if you're not hitting up against the damage limit).