Not Updated For Current Season
This is my first guide. I will spend more time revising the description later but I wanted to put the general idea out there for now.
Swain gains increased damage, healing and movement speed from his ultimate. His ultimate is his most damaging skill and the most useful for ganks and teamfights. He also looks like a beast. Therefore, it is advantageous to keep it up for as long as you can during fights. Unfortunately, it is very mana intesnsive and you will find yourself running out of mana at inopportune times. To counteract this, mana regen items are purchased.
The mastery includes a full 30% increased duration of monster buffs. This is because Golemn Buff is the lynchpin of this approach to mana regen. With a tear of the goddess, chalice of harmony and the golemn buff you are looking at 200 mp/5 base plus increased regeneration the longer you have your bird form active. With minion kills, this regeneration can easily exceed 300 mp/5. At this point, you will find that you can keep your ultimate up for 20 seconds or longer and still be able to cast your other spells. After toggling your ultimate off you will find you regenerate all of your mana in about 10-15 seconds.
Summoner spells chosen and specific items purchased are dependent on your team comp and whether or not you'll be laning solo. Ignite goes well with Swain's torment for an easy 20% increased true damage. Using clarity while your ultimate is active will give you a little extra breathing room early game but it is not needed. Keep in mind the point of this build is to keep your ultimate toggled as long as possible. It is very important that chalice of harmony and archangel/tear of the goddess are used.
Thanks for taking time to look through and check back later for a more organized and rigorous approach to my methodology.