Team comp for salter

Looking at what you have make an arts team with robin mash and grab a support Jeanne with prisma cosmos (MLB preferably)
You could use the default summoner gear.
Focus on making arts chains so that Jeanne will have her NP to survive salters NP and that mash could NP to offset some of the Def down that salter will start to inflict after the first break bar.
Not sure what the skill level of your servents are but its should go well.
Also not sure what CE’s you have available so equip robin with something that does NP damage NP gain or Arts up.
Equip mash with something that gives Def up or healing up.
Take advantage of mash’s taunt and mystic code dodge and or her invulnerability to soak damage for a turn and build up her NP.
Mash’s def up really does help in surviving the long run.
Good luck with the challenge quest

Mash/Robin Hood/Ruler Jeanne sounds good. Put either Divine Banquet (NP Gain) or Another Ending (Arts and Crit up) on Robin Hood and either Iron Willed Training or Divine Banquet) on Mash.

If you have to make a choice, focus on building Mash and Jeanne's NP instead of Robin Hood since the defensive abilities are more important to be able to survive. With enough Def Buffs on the party, it's possible to tank an NP and receive 0 damage even without Invincibility (Mash's first skill plus two of her NPs).

Robin Hood's Sabotage also lower's Salter's attack but use it to poison her right before you use his NP for maximum damage. With their resistance to Salter's attacks, Jeanne and Robin should have little trouble surviving unless Salter goes crit crazy on one of them.

And just in case, put Euryale, EMIYA, and Herc on your Backline. Hopefully they won't come out until her last bar and, hopefully, Mash and/or Jeanne are still alive.