Swain is best played as a counter-pick; never first-pick him and choose based on the enemy composition.
Focus on core items like Rylai's and Malignance to scale into a strong mid-to-late game carry, using Conqueror runes.
Utilize Ghost/Flash summoner spells, and for Magic Resist, Null-Magic Orb is a cost-effective choice.
Stats
Domination + Inspiration
Q
W
E
R