Myrrh

Myrrh's refine is great and keeps her relevant in today's crazy inflated meta. +5 to all stats and an improved pseudo-Wary Fighter effect makes her incredibly tanky.

IVs: +Atk/-HP is optimal. Her refine nullifies the HP loss, and whilst -Spd is still good, it isn't optimal. Hopefully you have some copies of a Gen I unit though.

Great Flame (+Eff): The bread and butter of this build. Her refine grants Myrrh +5 to all stats, nullifies follow-up skills, and makes it impossible to double Myrrh without NFU unless her foe's Def is at least 1 less of Myrrh's AND if her foe's Spd is at least 5 more than hers. Keep in mind Myrrh gets +5 to all stats as well. This is fantastic because a Def unit that doesn't rely on follow-up skills are very rare.

This makes her able to tank Red/Bow/Dragon effective weapons in a pinch as well.

Reposition: She's a flier, why not

Aether: Helps with Great Flame's HP requirement, with tanky foes and with sustainability in general.

Distant Counter: Enables her to counter against ranged units. Because dragonstones target the lower of Def/Res for ranged foes, this allows devastating damage against them.

Quick Riposte 3: This is another key skill that allows Myrrh to tank the single hit from her foe then DOUBLE against them. Because she can't be doubled, this means that her foes will only be able to hit ONCE whilst Myrrh will hit TWICE every time.

Joint Drive Atk: A very expensive skill, but synergizes with Great Flame extremely well. If Joint Drive Def gets released then that might be more optimal, but otherwise the C slot is very flexible.

Heavy Blade 3/Iote's Shield: HB3 helps with activating Aether, ensuring Aether will always be ready after a single round of combat. This is helpful for those tanky foes. Iote's Shield will nullify bows, but isn't needed because she will likely tank the single hit and survive.

I believe that this is the most optimal build for her. This turns Myrrh into a 187 BST EP monster that will never get doubled but always doubles her foes instead.

EDIT: Some more improvements to this setup.
This build is geared towards Myrrh's ATK and DEF stats and using them to deal as much damage as possible, while also reducing damage taken from enemies. Thanks to her having high ATK and DEF, it would be a no-brainer to give her Brazen Atk/Def, thus improving Ignis' damage output even further when her HP starts getting low. Seal Def/Res works pretty well in this setup by weakening their damage taken and improving the chances of Great Flame's passive effect triggering. If you have an extra Threat. Atk/Spd 3 to spare, this can greatly improve her offensive capabilities and has fantastic synergy with Seal Def/Res. If not, Threaten Atk 3 can be a decent budget alternative. Myrrh by default can be a bit of a glass cannon since she's both a dragon and flying unit, making her more vulnerable to units that have weapons that are effective against dragons (i.e. Falchion, Naga, etc.) and flying units via bows. This is amplified a bit more since her SPD isn't that good either. If you happen to have Iote's Shield available, having Myrrh equip it will make it easier for her to deal with bow units and could potentially nullify damage from them as well. Rally Attack is optional depending on what your teams consists of. In other words, adjust accordingly.
Handy tips: When Myrrh has Great Flame equipped and her "DEF is ≥ foe's DEF+5, foe cannot make a follow-up attack." Lower your foes' DEF or/and increase Myrrh's DEF if you want to use that passive effect to your advantage!

(Current Unit: Merged +5, Def Boon, +5 Dragonflower, and S rank SummonerSupport, HP: 50, Atk: 59, Spd: 30, Def: 46, Res: 36)
Ever since I pulled 2 Myrrh's the night she came out, I've loved her and she has never left my PvP team.
Her Great Flame prevents follow-ups when her Def is +5 to the enemy's, so I built her as a Def Tank.
Reposition is just great on a flier, allowing other team members movement over a 1 tile river by flipping them over her head.
Bonfire is just perfect with her huge Def.
Speaking of Def, her A skill in this build (worth it only if you already have 1 Surtr, then sac the second to Myrrh) gives another +8 Def when the enemy attacks, and has a Guard affect,
Giving her 54 Def upon being attacked, so anyone with 49 Def or lower only hits once and does not charge their special.
QP in the B slot allows her to double and kill many melee fighters.
Odd Def Wave was the only C skill I could find that buffed her own Def, so every other turn she gets up to 60 Def on enemy phase.
Then Atk/Def +2 just helps both her main stats.

This build is designed around using myrrh's great flame to it's max potential

Stats
Hp-42
Atk-46
Spd-22 (bane)
Def-49 (boon)
Res-31

I'm not positive if you can stack fort def on top of each other, if not use lotes sheild to prevent sneak attacks from bows

Even at 46 atk she can still put out damage and with their atk unable to beat her def she can tank most if not all physical attacks thrown her way

The C slot can be wiggled around for your play style but for max potential I recommend a skill that decreases foes def

I'm so lucky that Myrrh is great even with a bad IV spread. Though, I still need to make some adjustments and changes, and hopefully, I'll get a merge at some point.

Starting with her IVs, I have it bad. A Spd Asset is ENTIRELY useless. Thankfully, even with an Atk Flaw, her Atk is still decent, and she is meant to be more defensive anyway, so losing some raw power isn't the end of the world. Hopefully I can get a merge with a Def Asset, but honestly, ANY Asset aside from Spd will be useful in some way.

Her Prf is still viable, even without a Def Asset, so we'll stick to it. It comes with an Earthsweep Effect, allowing her to deny follow-ups from most foes if their Def isn't close enough to her own. There are a number of skills that circumvent this, so be wary.

As for Assist, due to her status as a Flier, Myrrh is better off with Reposition or even Smite, as with those, she can move her non-flying allies over mountains and rivers. Pivot is a decent option for a bulky character, though I'm waiting to get Reposition!

Bonfire is an excellent Spec as Myrrh's Def can easily reach 50+ with this build. More with a Ward Dragons buff, Tactics, Waves, or Fortify Dragons. Moonbow, Sacred Cowl, or Escutcheon might be desired if you want a faster Spec/more defensive capability. Aether might be worthwhile if you have a better IV spread, with some Atk boosting skills and Heavy Blade seal.

Your best options for Myrrh's A slot: Distant Counter, and Steady Stance 4. The former is a popular choice, and for good reason, as Myrrh is definitely meant for Enemy Phase engagement. The latter is better if you want to more reliably meet the requirements of her Prf's Earthsweep effect, with a nice Guard effect added in.

Myrrh's top choices for her B slot, are Quick Riposte, and Renewal. Quick Riposte is meant for more of an offensive presence during Enemy Phase, though you have to keep in mind, she isn't invulnerable, and she can easily drop below the HP threshold for Quick Riposte's effect. Renewal is really handy as it helps improve her survival in lengthy battles, like Tempest Trials. Can't go wrong either way, and bonus! They are both Seals as well, so you could easily run both at the same time.

C slot is somewhat flexible, though I went with her native Hone Dragons for now, as I currently don't have any other options for her. I'm looking into Atk Smoke to improve her survival, or Res Smoke to make up for her Atk Flaw. Goad/Ward Dragons isn't off the table either.

I went with Steady Stance seal. Because her Great Flame's effect is basically Earthsweep 1, she needs a LOT of Def to nearly guarantee follow-up denial. If I ever pull a Def Asset Myrrh, I might opt for Renewal instead.

Myrrh is a very effective wall, better than Nowi in my opinion, though as she is no longer in the general summoning pool, and only comes at 5-star, she will no doubt be difficult to merge. Still, she's strong, physically bulky, and even comes with decent Res. Her weapon, while it lacks DC potential, makes up for it by massively improving her survival in most match-ups. Myrrh truly is an amazing character, even with terrible IVs. After all, she is a dragon!