Swain is a tanky caster whose role is to provide cc and damage for the team. I almost always take Swain solo lane (top or mid) so I can gank quicker. He is easily countered by cc and ignite (because of his heal). Swain is my first and current main and I have made the perfect build after all my experience.

Here is my Match History :

I chose the 9-0-21 masteries but if you are planning to go more tanky you can go 0-9-21.

I picked 3 Quints of Health]/color] because in early game I don't pick a doran's ring so I make up with the +72 health from the runes.

The 9 MP marks are self-explanatory for any caster.

I took 9 mana regen seals because Swain needs to spam his spells to be viable and he runs out of mana fairly quickly (especially early game) so the runes help out.

I took 9 flat AP runes because we don't need the CDR runes because with masteries we have 24% CDR reduction and with spirit visage we have 39% CDR reduction which is pretty close to 40%. You could also take 7 AP and two flat CDR if you want the max CDR.

I pick Flash because it is a great spell that should be used on all carries (great for escaping chasing etc).

I pick Ignite for the damage output early game to help me get fed.

Early Game

I take meki pendant and two health pots for the early mana regen. I take torment at level one because it does the most damage and is most useful. I then go W because it keeps the enemy at half health if you are good at aiming skill shots.

I typically get first blood before level 6 by keeping the enemy at half health and when I hit level 4 I stun them - Flash (if needed) - E - Ignite - And q and autoattacking whenever I can, this almost always ensures a kill.

When I have around ~1300 gold I recall and get sorc boots for early mpen and tears of goddess so I can stack mana for late game.

Mid Game
I then get blue buff (ask jungler if you have one) then gank if I can.

After ganking I go back to my lane and farm till I can get a Mejai's (if you're fed otherwise get Evil Tome) and Rod of Ages.

Late Game

Get a Spirit Visage for the health regen because after the healing nerfs on Swain, this item is a must. Get the Deathcap for more damage output.

Now the team fights begin. To be useful to the team, during team fights stun the AP and/or the AD carries so your team can focus them. Throw torment on the squishy people and use your slow on tanks or on people trying to escape. Have your ulti on if you are being focused or if you have blue buff.

Finish the game (if it's still going) with an Archangel's staff or another survivability item
(Banshee's Veil, Rylai's, FoN, Thornmail).

Caitlyn - her huge auto attack range makes it so you can't hit your E-Q without taking damage. Against her I try to land my snare and use my E-Q combo and run back until I can finish her off.
Leblanc - her early game is insane and when she hits level 6 she can easily combo you. Try to finish her before level 6 if you can.
Malzahar - He is really annoying and can outclass you at level 6. Try to play passively and don't let him combo you or you are dead.
Mordekaiser - He is very annoying because of that shield which makes your DoT's useless. When playing against him solo just play passively because there isn't much you can do against him.
Morgana - A good morgana can outzone you, but if you can dodge her Q then you can beat her.