At first, mundo will do what he pleases. Hopefully you can keep him under control, and then there are a few things you must do. His Q has a low cooldown, and a low cost, so don't be afraid to just spam the ability, and i recommend keep range with your enemy. However, when you find yourself near an enemy, be sure to use E and W. Now when you build the Frozen Mallet and Rilai's Crystal Scepter, you must first land a Q and then get near the enemy as fast as you can, your W must be toggled on as well. Also be sure to max Q first, and then it is easy to poke down your enemy before you can engage.