The best thing to fight daggers specs is probably swords or whips/flails. Most daggers aren't cursed or flagged no_drop. And most dagger specs make use of their second spec to compliment their dagger spec, so they wouldn't want a cursed weapon. So flail works well, plus there are some lightweight flails available. Entwine does a good job of cutting down on the concealed attacks too. There are a lot of lightweight swords also, so if the RNG loves you, a couple lucky rounds of riposte + double thrust + flurry could very well win the fight. Exploiting vulnerabilities with swords can be easy too. Just remember, if you're going with iron, to have backup weapons. Most iron weapons are pretty heavy.
