Swain's combo is pretty easy, and he has basically one. You hit E + passive + AA + Q + W + R and walk among the enemies. When the ult bar is filled up and the enemies low, you hit R again and get a pentakill (not joking).

O combo do Swain é muito fácil e ele só tem 1 basicamente. Você acerta o E + passiva + AA + Q + W + R e vai andando no meio dos inimigos. Quando a barra da ult estiver cheia e os inimigos estiverem low, você estoura a ult e pega um penta (não estou zoando) :)

Twin shadows - Very good for Swain, because of the slow. You can chase and cheese.
Mejai's Soulstealer - Do it when snowball.
Rabadon's Deathcap - Do it when strong.
Shurelya's Reverie - Do it when all of your team is strong.
Spellbinder - Do it when strong.
Void Staff - Do it when the other team has MR.
Morellonomicon - Do it when strong and no MR in the other team, forget about the Grievous Wounds, Morello is a expensive item to do just for the counter heal.
Randuin's Omen - Do it when a the other team has an ADC strong or a strong crit.
Zhonya's Hourglass - Do it when you are strong, but you need some defensive. Swain ult still does damage when "Zhonied".
Thornmail - Do it when the other team is AA(auto attack) based and heals a lot.
Zeke's convergence - Do it when you are not strong, but someone in your team that is AA based is strong.
Locket of the Iron Solari - Do it when your team is weak or when the other team has a strong engage in team fights.
Spirit Visage - Do it when the other team has a lot of AP.
Abyssal Mask - Do it when you have a strong AP in your team.
Adaptive Helm - Do it when the other team has a continous damage (teemo, cassiopeia, kassadin)
Redemption - Do it when strong teamfights.