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.
Mumu: Jungle tank
Mao: Jungle tank
Skarner: Jungle tank
I build all my tanky champions to have roughly 200AR/200MR after which I build AP/AD items. This way I pose a larger threat to enemy and might lure them to focus on me instead of carries. If they don't I do annoying amount of damage. If the enemy line-up is heavy AD or AP I usually change one AR/MR item to something that fits better to the situation.