Swordmasters are crazy strong in enemy hands, once again though in player hands not that great. You see an enemy swordmaster surrounded by 6 guys annihilate them. Don’t make the mistake and try that yourself :P.
Edit: I’m talking about the background here. Duelists can be very strong but Swordmasters get old and then you can pretty much just throw them away.
As for tactics/strategies: Nets and polearms will save the day. If you have a guy with really high melee defense, engage him with shieldwall. If you can spare a second guy he should be forced to use riposte which brings him down to one attack per round and should give you enough time to deal with the rest before you focus him down with poles. Don’t waste your time and fatigue trying to kill him when he is not surrounded and/or caught in a net.
#1 rule is to ignore him when he does riposte, just put shieldwall up and wait your turn.
Generally, nets are great. Catch a hedge knight and dagger him. Throw it on a bandit leader and his overwhelm is disabled (and they get easier to overwhelm). They can also be used to keep the enemy from running (if fleeing) to give you more time to surround them and dagger their armor.
Or simply to have the enemy waste their AP to break free – e.g. enemy hedge knight with named 2h-axe that can probably oneshot most of your guys with a lucky headshot.
Also heard they work well against goblins. I’m not sure if they can be used against orc warriors and warlords.