If you're having trouble rooting an enemy with Nevermove, try using Decrepify to slow them down first.
While laning, try to gauge the strength of your opponents to determine the right balance of aggression with Ravenous Flock. If they're stronger, it might be more beneficial to stay back and cast Nevermove or Torment.
Swain can easily disable a single attacker. Coordinate with your allies when ganking him.
High mobility counters all of Swain's basic abilities: moving away from the Raven will break the slow, it is easier to dodge Nevermove, and you're able to escape focus fire while Tormented.