Myrrh

This build is essentially an upgrade to Myrrh's existing DC Enemy Phase build. This build has been tested at Arena 20 and AR 16.
[ Boons: +ATK/+DEF/+RES | Banes: -SPD ]
Really any boon can work on Myrrh. Other than HP, most serve to help Myrrh in the various areas she can spec in to, with +ATK pushing offensive builds and increasing healing sustain, +DEF being especially useful for increased tanking and, more importantly, increasing the threshold for triggering her prf ability of preventing follow-up attacks, which is huge for her decent speed that does not stop deadly sword users alone. +RES helps in tanking mages of course. +SPD should only be taken if going for a maxed Myrrh, and even at max it's not her best boon, so really it shouldn't be considered all that highly.
[ Weapon: Great Flame (+Eff) ]
No dragon weapon will do Myrrh more solid than her prf with it's refine. It basically makes lower defense units unable to double her under any conditions. With that, the only units that may be able to consistently double her would be both high defense and speed units, a combination quite rare. Add on extra stats with a lenient HP condition, and she becomes nigh difficult to K.O. in one round of combat outside of the strongest red nuking mages.
[ Special: Noontime/Aether/Bonfire ]
The major flaw in Myrrh is that while she can stop enemies from doubling her, she has no sustain. This usually means having to relegate a healing special like Noontime and lose out on burst potential. Aether is a stronger healing and offensive option, however fliers have few options getting cooldown charge without their A and B skills, leaving only Myrrh's seal slot. Unfortunately, you almost always want to run Iote's with DC, meaning that long cooldown specials are pretty tough on Myrrh. However, if you run a healer in your team, then Myrrh's special can be changed to a damage special like Bonfire that is perfect for Myrrh's good attack and defense.
A Skill: Distant Counter
The classic that gives Myrrh's mixed bulk and strong attack a chance to shine in her strong enemy phase against any opponent.
B Skill: Dragon's Ire
This is the skill that clinches this enemy phase build. Along with Myrrh's absolute stoppage of enemy follow-ups, now with the improved Dragon Quick Riposte she can always smash down an attacking foe with her own guaranteed follow-up. With a more lenient HP threshold and neutralizing effects that prevent Myrrh's follow-up, only a very small subset of enemies can attack Myrrh without getting bopped in the face twice.
[ C Skill: Pulse Smoke/Stat Smokes/ Atk/Stat Reins ]
Pulse Smoke is easily her best option when it comes to tanking, as stopping a deadly enemy special is essential to lasting on the battlefield. However, if you don't have that option, the easier to get stat smokes are fine replacements, with ATK Smoke for tanking, RES Smoke for dealing higher damage to melee foes, and SPD Smoke in the anti-NFU niche. Other alternatives are the new Rein skills for fliers, with Atk/Res being the most useful so far added and perfect for her build, or dragon/flying hones/goads for a Dragon/Flier Emblem team.
[ Seal: Iote's Shield/Heavy Blade/Smokes/Stances ]
Iote's Shield is easily to most recommended for taking on a lot of the nasty archers that have popped up, but alternatives such as Heavy Blade, the Smokes as mentioned above, and stances like Sturdy or Mirror can be used if Iote's is in use for a different unit.

Great Flame's unique refine makes Myrrh's speed easier to salvage and should be the main boon for her in general due to Great Flame disabling skills that guarantee the foe's follow-up while also blocking their follow-ups for having more defense then them, in this case it becomes a speed check. Unmerged Myrrh hits 34 with Great Flame's effect, 39 with Spd/Res Bond 3 and then 45 with a buff, on top of this you're sitting at an equal defense line of 41/41 on top of being quick with enough damage, making Mystic Boost much more superior so that she can stay as healthy as possible. If you want to tank archers then you can swap Spd/Res Bond 3 for Iote's shield, but this means you'd have to run Atk/Spd Rein 3 to cover the loss of speed while also being tankier, or Joint Drive Speed instead if you don't have the former.

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.