The debiru doggo or the tainted saint?

Cu's defensive abilities combined with Nightingale's heals *should* be plenty survivable enough for most content so long as you've got another def buff like Mash or Wavers backing it up I mean Cu's NP + def buff + attack down and Waver buff give you a 94% damage reduction for 1 turn.

Can't say 100% because Cu and Nightingale avoid me like the plague but at least on paper they should be plenty survivable with a defensive support backing them up. If nothing else you should never take damage from enemies NPs with this team comp so long as you maanage it properly.

Those 2 can work pretty well together. If you are worried about survivability, just bring in a taunt character. Leonidas would be great since he has buster buff as well, although his taunt is just one turn so bring a CE to help him charge NP up as soon as possible.
Gergorias is great himself with his 3 turn taunt, giving you higher flexibility with his CE.
The other servant you mentioned doesn't work that well with the team above. Jalter of course would work but is not necessary, plus Cualter himself can take on most high difficulty quest.