UR, due to Ragavan, can support a good counter even if it trades badly regarding mana and tempo
Bonus: every booster in that set came with this little gem
Thalia, Archon and Spellbinder are all quite good against them though, so you can definitely win matches against Mardu
The spellslinger archetype has plenty of depths, with token decks, burn decks, and storm decks all comfortably falling within the limitations of extracting the most value from your instants and sorceries (with some modern cards including other noncreature spells), so theres a little something for everybody
However, the biggest problem with the Emerald/Sapphire Azurite Sea deck is that it's a bit unfocused
Card Treatments There are quite a few card treatments to be found in Dragonstorm, so much so that it might feel a bit overwhelming