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.
Best combo in the game?
Amumu pulls onto an enemy (preferably when there are two or more enemies around) and Nunu runs into them with blood boil and starts charging his ulti. Then Amumu uses his ulti and BOOM. Then they can just try last hitting them with their other spells if they aren't dead already.