|Azura is one of the few dancers (singer) in the game, and not only is she a great supportive dancer, she also has the natural strength to kill red units.
To quickly explain the IV preference, +Atk and +Spd are never bad options. For banes though, we prefer -Res instead of -HP and -Def because if Azura ever takes any retaliation damage, it’ll most frequently be from a physical source of damage. We want to mitigate the risk we put Azura in at all costs, so it’s a much safer bet to reduce her Res as opposed to the other stats. Any IV set works with Azura though as she does her job regardless.
With an Atk/Spd stat of 31/33, she’s unable to handle common blues on her own and will have trouble surviving retaliation (provided these blues have high defense and high attack stats).Since she already has easy access to Sapphire Lance+ and is a blue unit herself, she can still fill the niche of killing reds though.
As a dancer you normally wouldn’t want her to be in harm’s way, so it’d be rare for her to engage in combat. This is what makes Fury such a strong candidate for her A slot. Although a lot of passives will work just fine for whatever you use Azura for, Fury gives her an overall buff in most of her stats. The usual downside to Fury is its chip damage, but that doesn’t matter too much because she’ll primarily be using Sing on a teammate instead of engaging in combat anyway. The attack boost from Fury is never a bad thing to have, especially when she wants to at least be capable of killing reds while Speed/Defense/Resistance are all extra layers of defense for if she ever does have to take a hit. Speed acts as a layer of defense because it prevents more units from doubling her; however, since it's providing only +3 to Spd, the same as her default passive Speed +3, it doesn’t help her double any additional red units. This leads into her B skill.
If Azura is going to be your primary and only red killer, she would most benefit from Swordbreaker 3 This is because certain key reds like Lucina and Ryoma don’t get doubled by Azura often, and will survive just one attack from her Sapphire Lance+. Forcing the double on the fastest of red units at the cost of her B slot is extremely efficient. Most reds, excluding the ones that have a very high defense/hp stat like Draug and Zephiel will flat out die to her double. Having Azura cover key threats like Lucina and Ryoma makes her a lot more powerful of a unit, though this is not necessary if you have other answers to these heroes. Thanks to Swordbreaker 3, she can even defeat merged sword units (eg. A +0 Azura can easily take on a +5 Lucina or Ryoma with Swordbreaker).
If you don’t need Swordbreaker 3, Wings of Mercy is a great option for Azura’s B slot. The ability to teleport to a unit to use Sing on them is really strong and makes Azura very mobile.
Her C slot is very flexible, but a buffing skill like Hone Attack/Hone Speed is strongly recommended due to the nature of this unit. Staying with/behind her allied units makes it very easy to buff your teammates with hones. Threatens shouldn’t be considered because she should rarely be in range for a threaten to take effect.
Her special is also flexibile as she will barely ever go into combat, but ideally you’d chose a low cooldown passive like Moonbow so that, if she does see combat, it will have a chance of actually seeing use and help kill an enemy vs. skills with longer charges. Other options include Escutcheon to add an extra layer of defense on top of what she already has.