Saber Alter Apparently likes to Smash Merlin...(and almost all of my supports)

For the challenge quest, Salter prioritizes casters. So yeah, Merlin is gonna take a lot of heat.

I ran Chloe, Mash, Hans, Merlin, Jeanne, Tristan. Just be sure to stack defense buffs as often as possible. Spam those arts chains. Use Mash's third skill when possible to take the heat off Merlin as well as mega-charging her NP and granting more defensive buffs. Try to use her NP second or third in an NP chain for additional damage negation.

Someone in the previous threads suggested double Merlin and David. I tried it out and it worked pretty well. I did have to use 1 command spell to refill one of my Merlin’s health, but only because RNG was merciless on me with the crits. I just made sure at least one of the Merlin’s invincibility skill or David’s party evade was available by the time her NP was charged.

Yeah, that's completely intentional. It's basically the gimmick for this challenge. "Kill the squishy and pesky support unit" XD

Merlin, in particular, is a quite squishy since he lacks any Def buffs of his own. Maybe give him a CE like Iron-Willed Training to give him a permanent defense buff if you really wanna use him. It's not a huge increase, but it does keep him alive longer.

Although Merlin can best support a Buster Servant, consider using an Arts type Servant or, at least, a Buster Servant with 2 Arts cards. This way, you're more likely to get Arts Chains, which increase NP gauges faster, which means less turns between NPs. Mash, especially, needs to have access to her NP as many times as possible. With enough Def buffs, especially Mash's NP and first skill, even Salter's NP will hit Merlin for 0 damage, letting you save one use of his Second skill.

Another thing you lack is a Servant who can drain Salter's NP gauge. With only 4 bars, it's not long enough for Merlin's second skill to come out of cooldown to grant team Invincibility. And double Merlin may not survive long enough, especially when Salter starts dropping Def down debuffs with every attack. To make it worse, her NP gives her one NP bar immediately, making the next NP come in only three turns. And every Break Bar removed will cause her to charge her NP by 1.

You might have seen the suggestion elsewhere already, but Euryale/Orion/Archer Arturia are prime choices to use since they can drain Salter's NP (Orion's and Archuria's are tied to their NP though and the NP drain is by chance and not a certainty).

Martha Ruler is an okay choice. Her first skill can't grant her ATK up since the battlefield isn't 'Waterside' but her Ruler class means she can tank one NP. After that, however, she'll need to get her HP up before she can take another.

Herc would be okay as well since Salter will ignore him anyway and his Evade and Guts can be used to survive NPs that you can't catch with Merlin's Invincibility. Though I think it might be better to have him in the backline as a back-up rather than the first.

Also, consider using Servants like Euryale, Robin Hood, David, or even Asterios.

Euryale and Robin Hood are NP nukes. Euryale can even drain Salter's NP gauge while boosting her own. Robin Hood can lower Salter's attack with Sabotage, making her just a little bit less dangerous.

David (Probably the best choice for you) has a 1-hit full party Evade that does not disappear until used up, can heal himself and the party, his NP might seal Salter's skills to keep her from buffing her attack or crit rate, and he also has 3 Arts cards, making it easier to form Arts Chains. His NP is easy to NP for max damage and is also a Buster-type, meaning Merlin can buff his damage by a large amount.

Asterios, like Herc, won't have any problems surviving since the focus will be on Merlin. His NP, on the other hand, can help Merlin survive longer. He can debuff Salter's ATK by 20% for SIX TURNS if his NP is at max level (which should be easy since he's a 1 star), and a further 40% for one turn if his Interlude was done. He also debuffs defense, making Salter easier to hurt.

Not sure what other Servants you have so I can't give any specific team comps asides from this. Hope it helps even a bit.

I used David / Tamamo / Merlin (support), no one died. I used Fox Wedding as cure skill, the second effect of the David's NP to avoid Salter's charge gauge skill.
The skills of Merlin and David to avoid Salter's NP or let merlin/Tamamo recuperate some healt.