Myrrh -spd +hp build?

-SPD, +anything on Myrrh is good! Her speed is already pretty abysmal, so no need to worry about dumping that stat. I would recommend Fury and Quick Riposte so she can tank and double on the counter-attack. As for the C-Skill, use Flier buffers or Hone Dragons depending on which team you use her on: dragon or fliers. Otherwise, you have some flexibility for that slot!

My Myyrh is -HP, +ATK, so I'm pretty happy with my IVs as well!

You can just slap on something like Vantage or Renewal and a C skill of your choice along with the QR 3 Sacred Seal and she'll be ready to go.