Build for Masked Marth

The build looks amazing in paper, but my only concern is regarding Wrath and how often she can take advantage of it, given her durability. I think a characters requires good durability for Wrath, that way they can take advantage of it every battle, rather than have to wait to charge their special and hope for a oneshot.

This build is going to be a bit inconsistent. If she can retain 25-ish HP after the first fight, she can usually take on a second opponent, and after that I think she's done. However, starting with 25 def, if her first opponent isn't an axe I would expect her HP to be 20 or lower after the first engagement.

Desperation will be a lot more reliable and makes a lot more sense to me with moonbow + flashing blade. If you're sticking to the build here, the thing that makes the most sense to me is dragon fang to really try to nuke the sh** out of anything if she can survive the second counterattack.

Ahh yes. Another person looking to build an underrated unit.
I'm in the same situation as you in having a Lucina with bad ivs and with Lucina being my favorite character I quickly went to masked marth to make up for it. I've been playing around with a good build and I've found that a mixed phase works wonders on her especially with DC. With QR as the B slot she will almost always use her special to finish off most things that hit her first.
My infantry team kind of supports one another so she also gets a bunch of passive buffs which pair well with the sealed falchion especially with baiting.
If you dont have DC fodder then you can always run a more offensive build especially with the new flashing blade seal while keeping that atk/spd brazen. Just like everyone has said, she's gotta survive the first hit.

Well, it's simple really
Fury activates Sealed Falchion, Vantage makes sure no one can touch her if she's abit low on health, Heavy or Flashing depending on preference reduces Aether cooldown, then Fury makes sure that even if she heals fully, Sealed remains in effect.
Atk Smoke makes other opponents weaker in their feeble attempts to kill her.

Also if you want to go the extra mile, might I suggest pairing her with:
Ally Support Yato
Spectrum Drive Falchion
Geirskögul
And let her allies have another full set of Drives resulting in a bonus +19 to all stats.

I think Masked Marth is one of the best F2P units in the game. +7 merge with decent IV's? Yes please.

I think a Brazen Skill along with Sealed Falchion is an interesting idea, but not completely optimal with Wrath/Moonbow. What you really want is for her to be either A. durable enough for PvE things or B. able to take out two units in Arena. Provided you double the first opponent you face, the second will likely kill you if it can counterattack. (which it should be able to, because moonbow is not charged on the second turn).

I'd suggest Aether as the special. You go below 75% on the first engagement, and with a double hit, ensure that flashing blade procs twice, charging Aether fully for the second turn. Now, Brazen Atk/Spd and Sealed Falchion are active, as well as Wrath, meaning you hit an Aether boosted ~65 attack with +10 from Wrath. Only things like Hardin or Effie will survive.

The added benefit is that Aether will most likely heal you back up to full. Or just back up to enough health to survive another engagement. Either way, you just killed two targets with her.

Tbh, Wrath in the B slot is a little overkill.

Also, Brazen Atk/Spd is not as good as Brazen Atk/Def on her IMO. She needs the def, whereas the speed will only be useful against Karla/Ayra, and even then you still may not have enough. And even if it is, those two matchups should either be done by another character, or you should one shot them with Aether.