Build for +spd -hp Halloween Myrhh

you can go with her best build (Distant Counter, Special Fighter, Quick Riposte seal). it's not that IV relevant aside from -def due to her weapon, in fact you can deny some doubles with 27spd.

or you can go full enemy phase with lightning breath(spd), double darting stance, and vengeful fighter.with 42spd, I doubt anything not named L!Marth can double her.

Spd is a superboon that pushes her into the 180 BST bin.

She can't really use the spd much, and she won't really miss the HP. Just consider her neutral and build as normal. She really wants DC if you have to fodder, but the precious DC fodder might be more useful on a more easily available unit that you plan to merge up. Don't ever give DC to a unit that is just going to sit at +0.

I just finished replying to another person who had the spd boon. (At the bottom of the link I provided - I provided already some answers there)

27 spd is actually workable. M.Grima +spd is also 27 spd, and if you add hone armour and some other spd boost (ex: darting stance seal) you actually avoid a lot of doubles which is sweet.

https://fireemblem.gamepress.gg/q-a/repost-i-pulled-x-halloween-unit-what-do-i-do