If blissey has ice beam, dugtrio is nothing, you can use 1 sei and 1 ice beam (dugtrio will use screen 2 times), so dugtrio can't counter blissey anymore.
First : Fissure will fail against a target Pokémon at least 30 levels above the user.
Second : Generation VII
Fissure's accuracy is now displayed as 30%; however, its accuracy calculation remains unchanged.
Maybe the accuracy remains unchanged so fissure missed? (= failed), i'm not sure about it.