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.
Swain is really easy champions to play. With few hundred AP you can kill almost any other champ with only 3 skills. First you want to root your enemies with "Nevermove" after that you have 2 sec to quickly use "Torment" and "Decrepify" ( And if able "Deathfire Grasp")
If you really want to get full Mejai Stacks just go in Mid-lane and farm minions for "Catalyst the Protector" and after that buy "Sorcerer Shoes" When finished buy "Rod Of Ages" for ability pover. Then go in to fight and make sure that you have enough mana to keep your ultimate "Ravenous Flock" ON for 5-10 Seconds. Then just buy the rest of the items and you will be LEGENDARY