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.
EARLY GAME: try to creep all you can to get gold as posible. If the enemies go near tower you could use your flash, to pulverize and headbutt them to your tower. Try to keep your flash to run away (alistar is very slow).
MID GAME: help your teammates, that is your work. If teamfights didnt start yet you could push yor lane destroying towers. In this part of the game you should had gotten the rod of ages, that would give you a boost of HP and AP.
LATE GAME: be always with your team. Alistar cant 1vs1 but he is necessary to **** them on teamfights with your pulverize to stop their skills like katarina, malzahar, nunu or other champions that canalize spells. You would help more your team if you had gotten your rylai's scepter. If u had a lot of assists and you got lich bane you would hit with yor ultimate activated arround 400 damage.
ADVICEMENT: If you are skilled, you could make your headbutt and inmediately press pulverize to push them and stun them at the same time. Watch videos on youtube of this combo.
Enjoy Alistar :)