Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
There are too many builds about Amumu as a pure tank, but I think that he can help as a DPS caster.
The combo rotation is quite simple, you must use Bandage Toss and Tantrum in early game for a high amount of damage.
Despair can't be selected in early game, because you're not a tank, you'll become a tank at mid game with your amount of Magic Resist (you can buy if you need it Chalice of Harmony at early game and sell it at late game, when you have enough money for start your Zonhya Ring).
Sorry for my bad description and the worse English of the world, I'll post results and change as quick as possible the description for a better understanding.