Best IVs for myrrh, planning to give her DC

The safest option out of all of them is +hp -spd cause since the other ivs have -res it's no a good bane for her(cause most likely you'll encounter Mages more than archers)
but if you don't care then...
normally people would recommend +def but....

From my experience +def is not necessarily better than +atk,
with +def indeed her weapon will be more effective but with +atk she will kill enemies more easily and even kill enemies in one phase/round when +def could not.
so if you want to kill more go with +atk but if you want to have the weapon effect to be more effective then go with +def.

but IMO go with +atk.

A flier with DC is still vulnerable to bows without Iote's shield, which I assume she'll have in seal form. And while she still has some respectable res with a bane, and that def boon is very high, +hp -spd makes her overall bulk that much better. I'd give her QR2/3 to help mitigate that low speed despite the IV's you use though.

If you're planning to high merge Myrrh, then you don't want -Spd as you'll want at least respectable speed on her to pass Bold Fighter speed checks against armors.

While it's not optimal, I'd take +Def/-Res as it's her best boon, and the extra defense will help avoid more doubles.

I would use +hp-spd

I would probably go +DEF/-RES. My +DEF/-HP is effectively the same thing since doubling just doesn't happen. Anyways mages can't punch through and since she isn't going to get doubled the hit to res is no worse than a hit to hp.

It does however make her more vulnerable to ploys and the Dire Thunder Siblings. However as a green she has Triangle against them and I am pretty sure mine usually survives with more than 6HP so it shouldn't affect this matchup either. So yeah while I wouldn't much care for -RES I would like the +DEF enough to eat the hit.