Best C-skill for Myrrh

threaten def or maybe def ploy with her decent res

Funny enough, that Myrrh's IVs was the first one I got as well.

So C-Skill without Distant Counter: assuming you just want her to tackle melee users, a threaten skill will be useful - either Res or Atk - depends on what you want her to do the most, more dmg or tank.

(Assuming you don't want any boosting/teamwork skills)

C-Skill with Distant Counter: I have used the following in my runs in the arena:

Def Ploy - pretty funny using this, since units with high def have low res.

Atk Ploy - didn't like this one since she doesn't get hurt from physical all that much, and magic units have higher res.

Atk Smoke - Currently the one I'll be using for offensive purposes, allows her to weaken the entire magical backline while killing the frontliner.

From all the arena runs I have been doing for testing, I would say Atk Smoke is the winner - just expensive to get.
Threaten would be the budget option that is consistent.

it's pretty situational given that I have a dragon team I just run hone dragons. def ploy/threaten def would synergize with her weapon well.

Myrrh has a lot of solid options for a c skill. Obviously she has two sets of emblem buffs, being both a flier and a dragon. For a mixed infantry and armor team guidance is a fantastic option, but that requires you to fodder a Tana, which is pretty expensive considering it’s a seal as well. Still, that would free up a seal slot for Iotes shield, which is fantastic to have if you put distant counter on her. Threaten defense isn’t such a bad option either. It helps her activate her weapon’s ability against high defense armor units like Amelia, though she will still be attacking into their resistance.