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.
So this is my typical amumu build. It works great if you're up against alot of Attack damage champs. If you are up against a champ such as Kog'Maw, or any other high attack speed champ, make sure you get a thornmail ASAP. If you're against alot of casters be sure to rush either a banshees or Force of Nature sooner rather then later, then focus on building sunfires. I always get heart of gold first, as amumu's build is very expensive and usually sell it when I need that last bit of gold for my Guardian Angels.
Almost all of amumu's attacks are AoE (area of effect) so with the sunfires it helps you maximize your AoE output. If you don't like ghost, flash is also a Pro spell for amumu!