How do I build a -Def Myrrh?

Either one works. I'd say Earth Boost for the +HP one or Fury for the +spd one. Don't sleep on that speed boon, though, although it's usually her best dump stat because of her weapon, it's a +4 and with Fury it turns into 32 speed, kinda decent to avoid some doubles from middling speed units so you don't have to worry about depending on your defense to avoid the double attack. Units that would be able to double her probably won't be able to anyway thanks to the weapon effect(for instance, Ryoma doubles naturally but only has 30 defense with Fury, he won't double unless he's +def and not -spd, something similar happens with Zelgius, he can only double if he's +spd and has some sort of speed boost). This doesn't happen everytime(a neutral def Ayra could still double), but I can see it catching someone by surprise.

If you still want to use the +HP one, just run Earth Boost if you can. It's a +6 to defense whenever it's active compared to the +3 from Fury, and it helps capitalize on that boon. +HP seal can help her activate it against more units if you don't want to run Iote's Shield on her(maybe because you already have a user for that seal or just want her to duel melees).

I have one that's +Res/-Def, so I'm trying to wait till I get something able to learn Lightning Breath+. I'll inherit Lightning Breath+ to Myrrh, slap on some stuff, and then use her to counter Mr. Magic-Is-Everything and some other Blue units common in the arena. However, since you don't have the +Res, it's maybe not quite as useful, so I'd recommend trying the Earth Boost on the +Hp one.