I don't know I am still new about summoner level 16 but It seems like fighting him is so one-sided. He will get tons of minion, he will get tons of items, he will one shot you, He will carry the entire game and, there is nothing you or your entire team can do about it or at least, that is how it feels.
Only way is to go an AD assassin and buy executioner's. He will just start MR vs. any AP champ and be unkillable. Other than that Swain has no real counter in lane.
Have high mobility to be able to move out of his abilities before they do too much
Long range poke, Morellonomicon/Executioners/Ignite, and building some MR. It takes practice but Swain for sure is rough to play against.
Mortal Wounds and Ignite. Take advantage of the cooldown on his ult once he turns it off. You have a nice window to do some damage before he can turn it on again. Also, try not to fight him near your own minions, because he can heal off them. Anything to reduce his healing will help.
But a good Swain is really hard to beat, which is why he is banned so often. He's relatively easy to play, and extremely strong. I hate him with a passion.
