Advice on Challenge Quest:

The first time through, I won with Mash (MLB Iron Willed Training for extra defense), Tamamo Caster (same CE as Mash, yeah I actually have two of those MLBed 🙄), and Support Archer Arturia (she had Cute Orangette to boost NP Gain and NP Damage). My Mystic Code was the basic Chaldea Uniform (heal with 7 turns cooldown at max level, an atk buff for Archuria, and an Evade just in case.) You can try this line-up. But with Tamamo there, she'll be tanking pretty much all of Salter's attacks.

Tamamo's Curse and Archuria's NP can both drain Salter's NP, but the NP only has 60% chance so don't always count on it especially if the NP is already full. If the NP is full, I suggest using Mash's Invincibility or the Mystic Code's Evade on Tamamo to keep her alive and saving the NP for one more turn. Because of you use Invincibility but the NP drain hits, your Invincibility is wasted.

Focus on keeping the Def up buffs active. Archuria's Arts Buff also. Comes with a full party defense buff to add to your defenses. If you manage to activate Mash's NP twice, then add her first skill, it's actually possible for the whole party to tank Salter's NP with enough defense buffs. I managed to do it twice and the NP hit for 0 damage both times.

Tamamo's NP is the key to NP spamming, especially Mash's and Tamamo's. Given the choice, focus on defense rather than Archuria's NP. Tamamo's NP, her third skill, and the mystic code's first skill are the only targetable heals available (AOE for Tamamo's NP). With her class advantage and the defense skills, Archuria could actually tank the NP with no difficulty then heal with her second skill (this way you don't have to worry about giving her the Invincibility or Evade). So you'll likely be using the heals on Tamamo most of the time, and a few times on Mash.

Mash's Taunt skill comes with an extremely powerful NP gain buff. Use it when she had lots of Def buffs up AND she has 2 Arts cards on the line up. This will fill up her NP gauge pretty much almost instantly. And it will keep Tamamo damage free for one turn.

Can't remember if I'm forgetting anything else, but I won with this line-up and strategy. Though my Tamamo's skills were at 10/8/10 and the Archuria I borrowed was 10/10/10 with NP lvl 2. Mash and Tamamo died with 300k HP left (their HP was slowly whittled down over the entire fight) but I had Orion, David, and Euryale waiting at the backline so I didn't have trouble with the last 300k health.

Try it out if you want and see if it works for you. Good luck.

I use tama(CasGil) jeanne and Archuria and the only thing to do is protect tama at all cost.

I completed it with my Merlin, Merlin support & my Kuro. If you can avoid one of the supports dying early by unlucky crit/focus by SAlter and get the NP buffs up + Kuro's crit down debuff you should be able to do it easily. Additionally, SAlter will charge her NP bar by 1 after each break, so best to do break her either when she has a full bar or just after she uses her NP. I didn't know this the first couple times and thought she charged her NP randomly and ended up getting wiped.

You haven't mentioned what supports you have but I see you have Tamamo if not Merlin. She will work but you must know that if you only have one caster on the 'field' then they will get focused down by SAlter. You need to run two casters so they can share the beating, or none at all. Good luck.

Bring almost all of your strongest acher there and for the team comps, here's the example :
Squirtoria + friends David + Mashu can do the job. All of party members should use survivability CEs like Steel Training or Guts or something that can increase your staying power.
The benefits of using def up CEs for your archer there, they have base resistance to saber - 50 % damage and you've double MLB Steel Training/Iron Will ( def up + 20 % ) and that's nice.
Combined with your Squirtoria party def up, you can at least mitigated Salter NP damage or her base atk up to 85-90 % ( depend on your skill level ). By using Mashu NP and her party wide Def Up skill, Salter NP or Crit or mana Burst is a joke.
For max result, use Atlas Academy MC. It is the best MC for free quest, harder contents and solo battle.

Put Emiya or Chloe as a Last Man Standing.

This game can be cleared by using 1-3 star, even Salter challenge quest.

For now, i've cleared Salter memorial quest without using CS or SQ twice and i'm even help my sister clearing it so that would be for the 3rd times i've successfully kill Salter there.

1st clear, hard mode : Gilgamesh + friends Merlin + Mashu. Merlin is totally a burden there, i barely win this.

2nd clear, easy mode : David ( Steel Training/Iron Will ) + Tristan ( Steel Training/Iron Will ) + friends David ( Sakura Valentine CEs, no survival CEs equipped ). I can tank all of Salter NP and some of her attack as well by using sheer short CD party wide Evade and + 50 % own Def Up from David 1st skill and also reduce damage - 65 % each time Salter attack my David or Tristan. Those 3x party wide evade also has decent amount of heal, which is nice.

I hope this can be a little help, remember to survive the challenge quest and not to recklessly rushing yourself to finish the challenge quest.

I did it with a stall team: Mash - Tamamo - Merlin (Sup). And the Atlas uniform.

Tamamo helps control with her first spell and if she fires her np twice inbetween excaliblasts you can use Merlin's invulnerability on every enemy np turn. Merlin needs his second spell maxed for consistency.

Mash helps counter the nasty defense debuff and since she never gets targeted in a pinch you can throw her invu and the atla's one at the other two and tank the excaliblast. She easily survives with either her snowflake shield or her np's buff up.

The reduce cd from the Atlas uniform also helped me when the game trolled me with cards, Tamamo's first skill don't working and other shenanigans.

Took a long time, but it was my first time trying a stalling strategy, was somewhat refresing from one shoting everything on each wave.

Gonna join the arts stall train here. I ran Archuria/d'Eon/Tamamo the first time I did this and Robin/Mash/Tamamo the second time. Robin's Sabotage is basically a 15% def boost to you, so between Robin's Evade, Tamamo's Morph, and Mash's NP you don't have to worry about getting NP'd at all. The trick is spamming Tamamo's and Mash's NPs as often as possible.

You definitely have David and you definitely have Mash. Getting Mash to 10/10/10 should be your biggest priority right now if you don't have her there. Raising David and leveling his Harp of Healing (ideally to 6, but 10 is better) is also a good use of resources.

Mash + David + Merlin can do this quest pretty easily until the final bar---Gamepress updated the section and detailed the team. Go read through that and see if it's achievable for you.

The biggest thing is---you KNOW Salter is going to focus on your caster. A good comp will play around that.

Mash, Jeanne and Merlin max skill max level seems like a very common stall team on this forum. And it works for me and a lot of people so you can do that.
Just spam arts and let it rip. Remember to protect merlin at all cost, but be wise about using your skill. Have archers on the back line in case one of them die in the last 20k red bar.