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.
The item set works perfectly but depending on who you are playing against you may want to get Mercury's Treads instead of Sorcerer's Shoes. Also once you have a lot of money and are finished with the build sell Shurelaya's Reverie and buy another Rabadon's Deathcap.
Swain can be made very OP if you want him to be. In order to obtain the best results using this build you will want to play the solo lane on either 3v3 or 5v5.
Exhaust is essential when using Swain as explained in Skill Sequence. Flash on the other hand can be exchanged with whatever you want but Swain is kinda slow so I would suggest something that can be used for running from or chasing your opponents.
Blue buff is great with Swain. The combo of the blue buff and Swains ult is amazing he can stay in bird form for a very long time leaching health and dealing crazy damage.
When entering a battle 1v1 I would use the Sequence of ERQW but if the opponent is low on health or runs a lot pop W first to keep them in place while you kill them. When your opponent gets to under 2/3 health use Exhaust so they cant run and if they do you can catch them. Also for a good surprise attack flash in and use exhaust then use REQW combo to take them out.
When entering a battle with more than one foe start with your R (ult) then walk in and use WEQ combo. Be sure to not accidentally tap R 2x or you will most likely die from your ult not keeping your hp up.
When farming minions especially in large groups start by using W to take out most of their health then R to take them all out quickly after.