[Challenge] Save Lachesis

You can't, especially if she is +Spd/-Atk. It's the worst IV set for her.

It's not like I'm saying anything new but Staff users really depend on every little bit of optimization they can get in order to be effective despite their low BST.

Lachesis can't be saved because she is suboptimal version of Genny, at what they both do best (colorless mage). So, in order to save Lachesis, you need to SI Wrathful Staff 3 to her, which isn't smart since Genny is superior at using it.

That being said, a while ago I was thinking over a double theorycrafting for Lucius and Genny, giving 2 possible effective builds for them, in order to give Staff users a bit more use among the players that are willing to use them. I could finish it up if I get the inspiration to type out the rest of the description x)

+Spd is actually fine as it'll help her avoid getting doubled by a couple of units.

The biggest problem with most suggested cleric builds is they involve Dazzling Staff (available on one LE unit) or Wrathful staff (available on one 5 star only unit). That said, Lachesis is one of the better options for inheriting Wrathful Staff if you have an extra Genny, but I probably wouldn't do it with a -Atk Lachesis. I'd suggest a more standard build with Live to Serve (the level 2 version is plenty enough and available on 4 star Wrys), Rehabilitate, and Heavenly Light.

Just make a standard healer build out of her and use her as an extra in arena assault. Give her plus 3 speed and wings of mercy or live to serve with rehabilitate.

In terms of using her in arena, just don't think about it. Most would tell you that it isn't worth the investment to make a "colorless mage" with bad IVs. However she will do just fine supporting units in the training tower, since she will just heal, however any healer will do the job, so she isn't all that necessary.