So, this is probably the only guide I'll ever write.
Consider yourself lucky.
Some profanity shall follow.
Swain is a tanky *** mage, adept at bursting down whoever is unlucky enough to lane against you, while surviving just about anything.
Early game, focus on getting cs, while throwing your dot (E) on your opponent whenever they venture into your range. Don't try to engage unless you're getting a gank, or the other bro is a ****** and wants your *****, in which case dot the ****er down. Your E combos with ignite insanely well, as it increases ALL damage you do to whoever has been hit. (auto attacks, spells, summoner spells)
Post ult/mid game, you can start harassing more frequently, just be careful to watch your mana. While you're pretty efficient with your passive, your ult will drain your mana in no time if you aren't careful. Get a chalice early if you need to. Once the team fights start, try and get in the middle of the fight, as your ult will give you monster sustain, and focus down the enemy carries.
Random tidbits:
The hardest part of playing Swain -
Using your root correctly, and knowing when to engage/what you can survive. Don't overestimate yourself. While you're pretty much the best mage in the game, you can be killed.


I'll probably add to this later, feel free to leave comments/suggestions/cries of heresy.