Who is most suitable to become a colorless infantry mage?

If you are focused purely on Offense, then you should try a regular Genny build with Atk+ Hp-, Atk+3 or Death Blow, Wrathful Staff (already available), and Savage Blow to chip or a Hone/Fortify. She should equip Assault and have Rehabilitate as a support skill and Kindled-Fire Balm as a special skill. If you do not have Genny, who is the only fodder for Wrathful Staff, then it will be very hard to make an offensive build out of a healer. Although, I am not sure what team you are currently using, so this may not appeal to a defensive team, but for pure offense, Genny is the best choice as of this moment. If you need, I can also give a support healer build.

You could try Elise on a cavalry team with wrathful staff. She has strong base offense stats 32 atk/32 spd and cavalry buffs would make her damage absurd for a healer. Would go for +atk or spd of course, preferably -def, but the other stats dont matter too much either.

For the A-slot, Atk +3 or perhaps defiant attack if you think you can get her hp down with reciprocal aid or something. Unfortunately death blow and the other good stat boosting skills cant be used by staff users.

But as mentioned before you need Genny either way and she is pretty good with wrathful staff herself .

Well, I haven't built one of these yet, but... *cracks knuckles* Here we go!

WHO:

Anyone upside 30 attack will perform best. Those with 28-ish attack can probably work too, with attack boons.

I would look at Genny, Lucius, and Serra first, as they all have good res and/or good speed. Genny also comes with Wrathful Staff already, so she's the most budget friendly.

Other options would be the following with +Atk, -HP or -Def IVs: Bridal Lyn (if you have her), Maria, and MAYBE Sakura. Lyn and Maria have the speed for doubles if you give them Spd+3.

BUILD:

Weapon: Assault is a must.

Assist: Whatever suits you. Martyr might be interesting, since this cleric will be seeing real combat for once.

Special: I THINK healers can still inherit regular specials -- if so, try Iceberg or Luna. If not, stick with your favorite healer special or Miracle.

Passive A: Choices are limited for healers, unfortunately. Atk+3 or Spd +3 are your best bets. Or you could try Res+3 on someone like Lucius and watch them become the most infuriating magic wall on earth, lol.

Passive B: Wrathful Staff is mandatory, I'm sure you know.

Passive C: I'd try Savage Blow, Threaten Res, or Threaten Speed.

That's all for now. Apologies for the long post.

I hope this helps, and good luck!

Im thinking maria might be one of best candidates if you include a 4 star +10 merge because of her high speed would probably allow for some doubles resulting in higher overall damage. She would hit like 38 spd at +10, 41 with +spd IV