The deck works just like any other U/W Flyers deck with Sephara - winning by getting her on the battlefield protected by Siren Stormtamer or rushing them down with Rally of Wings, Favorable Winds, etc. However unlike most other lists I've seen online this one took some ideas of Mono Blue to make it more annoying (mainly Curious Obsession and counter-spells). By having access to white it succeeds where Mono Blue often fails in Bo3 (given options like Prison Realm or Hanged Executioner to exile threats like Shifting Ceratops).

8 out of the 12 creature one-drops are pirates which is why Lookout's Dispersal is the counter-spell of choice. Being able to play it after a turn 1 pirate makes it more often superior to the other choices. It can deal with both T3feri and Risen Reef when your opponent goes first. In general, people are very rarely able to pay the 4 mana cost and if they are anticipating a counter-spell they usually just hold 2 mana open. With no pirate onboard most of the time it's still a Sinister Sabotage just without the surveil.

Mulligan:

With this deck you obviously always want to go first.

Having at least 1x blue mana, 1x creature one-drop and 1x Curious Obsession is the ideal keep. Otherwise I keep 2+ lands, 1x creature one-drop and 1x Winged Words or Lookout's Dispersal (if you got a pirate).

Every other hand I mulligan with exceptions like 2+ lands and Cerulean Drake vs. red or 2+ lands, a creature one-drop and Devout Decree in Vampire match-ups.

Sideboard:

• Devout Decree: This deals with vampires, Marauding Raptors, Runaway Steam-Kins, Chandras, etc. Since it's mostly included against aggressive decks I usually swap them for offensive stuff like Rally of Wings or things that are too slow or too vulnerable (e.g. Curious Obsession/Hanged Executioner against RDW).
• Cerulean Drake: Include this against any deck with red creatures or red removals (e.g. Gruul Dinos, Feather, RDW, Temur Elementals). I always swap them for the Faerie Miscreants.
• Narset, Parter of Veils: Run this against Field of the Dead, Esper and Nexus decks. They have a hard time removing with it.
• Prison Realm: Inculde this against any deck with green. Don't wait for until you see them play the Ceratops. If they are playing green they have it in the sideboard.
• Teferi, Time Raveler: Very strong against Simic Flash & Nexus.
• Mu Yanling, Sky Dancer: Very strong against slow decks that don't run many creatures like Simic Nexus & Esper decks. You can keep doing her +2 ability until you get her -8 ability that lets you draw cards if you tap islands.

Strategy notes:

You can use Rally of Wings defensively as a surprise against creature-heavy decks after you have attacked, since it not only buffs but also untaps your creatures. With this in mind, you can also use it to untap your creatures when you're attacking making it possible to attack for a lot of damage and play Sephara on the same turn. Also keep in mind that it's not always worth to play Spehara the moment you can. It's often better to wait a turn for left over mana with Stormtamer or a counter-spell.

If you're on turn 1 with multiple creature one-drops and wonder which one to play first; don't play Siren Stormtamer - rather keep it to protect another creature with Curious Obsession. Play Spectral Sailor (at the end step of the opponents turn) over Faerie Miscreant for a possible Lookout's Dispersal followup.