This build is made for lane sustain against ANYTHING and for raping and surviving teamfights.

Swain hungers for tons of mana (especially the ultimate), that is why you should have the Mana Regen runes, purchase the Doran's Ring as starter item and purchase the Tear of the Godess which you upgrade to an Archangel's Staff. You would also want to obtain the Blue Buff every now and then, especially before team fights.

You won't have insane amounts of Ability Power using this build, but Swain is not made to nuke the sh*t out of everything in one burst like Veigar is.

You're going to poke the enemy team to death and not lose huge amounts of HP as you do that.
Poke them down to about half HP with your Nevermove and Torment, then burst them with all of your abilities combined into one chaotic scene.


Effective minion farming:

Early game - Last hit with your basic attack. If you have the mana/mana regen you could use your Nevermove every now and then.

Mid game - Use your Nevermove and activate your ult to pick them down.

Late game - Nevermove. Nuff said.

Try to gather up as many as you can in your Nevermove, target their main damage output with Decrepify and Torment, then run around them with your Ravenous Flock activated. Continuously cast your other spells to ensure them to die.

If some of them start running, chase them.
Use your exhaust to close in on them and make sure to hit with your Nevermove, then use the rest of your abilities.

Are you being chased? Activate your ultimate. With the Will of the Ancients you will restore so much HP that you are most likely to get away alive. Using your Decrepify is also very effective because of the slow.