How to counter and fight a Swain

Recently, i fighted against many Swains and even if I camped them as a jungle to get about 5 kills, he still at late game melt I and my teamates while getting all his health back. Can someone tell me how to fight some fed/not fed Swain ? Because when he is full build he doesn't need blue buff anymore because he has too much mana. He does more damage than a ADC and is more tanky than an usual tank. Thank you for the help ! <---- annoying as hell.