Ishtar

Boons: +Spd/Atk
Banes: -Def/HP

A: Swift Sparrow 2
B: Desperation/Swordbreaker/Lancebreaker/Chill Spd
C: Odd Atk Wave 3

Support: Reposition/Ardent Sacrifice/Dual Rally
Special: Moonbow/Glimmer/Luna

In terms of IVs, Ishtar might be the first mage in the game that doesn't have to fall back on bladetomes with a -atk IV, or sit on the bench forever with a -spd IV since Mjolnir and odd atk wave boost her offensive stats to absurd levels regardless. Initiating with 46 spd with neutral speed is 1 point shy of the likes of +10 merged +spd Ayras and Mias, which means that she doubles the vast majority of the cast in the game.

With this in mind, her legendary tome is a no-brainer. Darting blow 3 + slaying effect on a 14 mt tome is probably one of the strongest offensive combinations one could ask for, and Ishtar makes great use of it with her fantastic offensive spread of 34/36. The slaying effect, in particular, opens up a ton of options on the specials. Glimmer, generally speaking, will out-damage moonbow by a pretty significant margin, and has great synergy with odd attack wave. However, Moonbow will begin to reap more value if you're facing more magically bulky enemies.

If you're facing more distant counter units, however, Luna will fit the general flow of battle much smoother than Glimmer or Moonbow. Attacking and then getting counterattacked will leave you with a charged luna on your follow up attack with Mjolnir, unlike Moonbow or Glimmer which will have 'wasted' a special charge when the enemy counterattacks.

Swift Sparrow is also a no-brainer here as it has the best synergy with Ishtar's tome and C skill, giving a massive +10/+10 to offensive stats on odd-numbered turns, and +4/+10 otherwise. Thunder Goddess indeed.

Thanks to her extreme speed potential, Ishtar is also one of the best users of desperation as her ability to double is almost unmatched. If you're facing a lot of speedy swords or lances, Swordbreaker/Lancebreaker can help secure those matchups better (although, do be warned that a Slaying Edge/DC/Wrath Ayra will most likely destroy Ishtar immediately on the counter-attack).

We keep Odd Atk Wave as it boosts Ishtar's offensive powers considerably on odd numbered turns without having to rely on support buffs.

Ishtar overall is just a fantastic mage that manages to carve herself a niche amid the highly competitive blue mage pool of Linde, Reinhardt and Lute. While she doesn't really have the offensive burst of bladetome Linde and Reinhardt, and doesn't have quite the res to serve as a powerful mage tank like Lute, Ishtar offers unparalleled consistency in her damage output thanks to Mjolnir.

My favorite FE character ever is obviously Summoner supported. The goal is to get off Aether like crazy so she’s supported by Infantry Pulse as well. Chill Res gives her an effective 61 Atk against its recipient with Swift Sparrow factored in.

IVs:
Ishtar is a unit that does not have bad IVs, only better IVs. Even at the least favorable IV (+def/-atk or -spd) she still has enough atk and spd to make her a decent unit.
As an all out offensive mage unit, her def is the go-to dump since even with +def IV, she would not be able to survive much hits. +SPD IV makes her spd stat goes up to 39 during enemy phase and 49 during player phase (with her weapon and swift sparrow). She is able to double almost every unit in the game that is not heavily merged.
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Weapon:
Her divine weapon suits her the best. It is essentially a slaying Tome with enough might and darting blow build in.
However, with her high atk and spd, a standard blade tome build can also be used on her.
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Assist and Special:
Ardent sacrifice can be used to get her into the desperation range. The same goes to reciprocal aid, but if your team has more than 1 character that has a lot of HP (greater than 40), you might want stay with ardent sacrifice since for Ishtar to proc desperation, she needs 30 HP or lower.
Drawback in my opinion is the best assist to all glass cannons but with drawback, there are no reliable ways to proc desperation. So use with caution.
For arena rating purposes, a dual rally skill will work as well.

Moonbow in my opinion is the best since she has a really high speed and her weapon allows -1 CD to her special; allowing her to proc her special almost every turn.
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Slot-A:
Her default swift sparrow is one of the best offensive skills. No change needed here.
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Slot-B:
Desperation is better than vantage since she is better at player phase due to her weapon and slot A skill. There are also too many distant counter units in Arena. On top of that, she is also not likely to kill members of cavalry, flier and armor emblem teams in one shot since these teams will almost always carry fortify/ward armor flier and cavalry buffs, making killing a unit in just 1 hit more unlikely.
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Slot-C:
Her default set allows her atk to reach 51/54/57 during odd turns (if you count the +4 from swift sparrow it is 55/58/61 during player phase) which is fantastic for an offensive mage. This also buffs surrounding ally which makes her a good support unit. If you got her as a -spd/+res IV, you can also use speed ploy here.
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Seal:
Quickened pulse combined with her weapon allows her moonbow to be charged and ready at the start of the turn 1.

Distance defense 3 allows her Res to go up to 29/32/35 and def to 19/22/25 while attacked at a distance. This can help her to survive more hits.

Speed +3/ Attack +3 can be used if you got her at a -spd/-atk IV.

(Stats are from my own Ishtar of +Res -Def with Weapon and skills and seals included. Optimal Ivs are either +Atk/Spd with -Def.)
The Goddess of Thunder cracks the skies upon her arrival to Fire Emblem Heroes. Ishtar arrives to remind us that Magic truly is everything.
1. 34 Atk and 36 Spd ties Ishtar with Linde for the best offensive stat spread for Blue Mages. Despite having 1 Less Atk than Linde, Ishtar's tome more than makes up for it, we will discuss about that in just a minute. A small, but visible thing Ishtar has over Linde in terms of stats is physical bulk. While Ishtar's physical bulk is bad to begin with, it's not as bad as Linde's where she vaporizes to any physical attack that comes her way; it allows Ishtar to maybe survive a physical hit if need be.
2. Her tome Mjölnir combines the Slaying effect with Darting Blow 3 to create a frightening Player Phase. Ishtar already comes with Swift Sparrow, combined with Mjölnir, Ishtar reaches 52Atk and 46 Spd. To put this into a perspective, with just her default kit with Neutral Ivs, Ishtar doubles the entire cast with their default skill sets with a few exceptions.(Lonqu who barely denies the double with 42, Shigure with Darting Stance reaching 43, F!Robin with Blue Tome Breaker, all Armor Units with Wary Fighter by default, and L!Lyn with her unique skills if the conditions are met.) With +Spd Iv or Seal, Ishtar will always be able to double naturally even against High Spd units with their optimal sets, making her the best user of the Desperation skill.
3. As if that wasn't enough, Ishtar also comes with a new skill called Odd Atk Wave. This C-Skill is quite tricky to use efficiently but it is powerful. At the start of every odd numbered turn, not only does Ishtar boost her allies Atk by +6, but she also boosts her own Atk by the same amount. This combined with Swift Sparrow and Mjölnir, on Odd-numbered turns, a neutral Ishtar initiates a combat with 58Atk and 46Spd which is insane. If the first hit didn't kill her target, the more than guaranteed Follow-up with Moonbow charged will.

Ishtar has certainly set the bar high for later to be released Mages. Her Player Phase is devistating, and in my opinion, not to be underestimated. Ishtar joins the ranks of Reinhardt as one of the best Magical Nukers in the game, and once again my personal opinion, competes with him, Lute, and Linde for the position of the best Blue Tome Mage currently in the game. While Reinhardt is more oppressive thanks to the extra movement cavaliers have, Ishtar is not as vulnerable as him in the Enemy Phase, thanks to her Spd and just enough Res, and her support and self buff properties with her new C-Skill(But not at the level of Linde and Lute though, who their team supporting abilities put them at the top of my list for best mages. Lute gets more points for being a Magic Tank as well.), all the while being a Player Phase powerhouse just like him.

Merge: +0
Seal: Squad Ace I 3 (Spd+3)

Player Phase Ishtar

Pros:
- Amazing standalone unit that hits incredibly hard on odd-numbered turns
- Gains Spd+10 on every initiation she makes, in which she can easily avoid getting doubled by the likes of Ayra*
- On odd-numbered turns, she gains Atk+10 if she initiates combat and usually 1HKO's any kind of Red unit that severely lacks Res
- Amazing out of the box so long as her offensive stats are not tinkered in a bad way
- She is the first unit ever to possess a tome with a permanent cooldown acceleration (which makes it good for triggering Glimmer/Moonbow when she's under the effects of Desperation)
- Very straight forward unit that requires minimal skill inheritance

Cons:
- As one might expect from a fast, hard-hitting mage, she is physically frail, so keep her away from Hector, Poison Dagger and Kitty Paddle users
- Her overall bulk is rather mediocre at best and seemingly resembles that of Nino's and Tailtiu's

*Although, there is a very slim chance for Ayra to survive any kind of Blue Mages in general, but if she does in any circumstances, Ayra's standard optimal build can easily revenge-kill Ishtar