Dimir Self-Bounce is entirely blue and black spells with heavy bounce and disruption like Boomerang Basics and Fear of Isolation, so Mystical Dispute is a premium cheap answer to their many blue spells, since it costs only {U} when it targets a blue spell. Spell Snare only hits mana value 2 exactly, and much of their disruption like Thoughtseize, Fatal Push and Boomerang Basics costs one mana, so it is the weakest of our counters here. Yorion, Sky Nomad is a 4/5 flyer that can blink our own ETB creatures like Beza to get their value again, though it is still vulnerable to their bounce. Keep Dovin's Veto and No More Lies to stop key noncreature spells like This Town Ain't Big Enough.