Therefore your best bet for this achievement is clearly the Flagellant not only because of the higher odds of success but because he becomes significantly stronger while at Death's Door(+5 ACC, +20% DMG, +2 SPD, +20% Stun, Blight, Bleed, Debuff resist) whereas other characters receive a debuff(-10 ACC, -25% DMG, -5 SPD, +33% Stress). The Stalwart virtue adds +8% death blow resist in the event you do not have a Flagellant as they can never becomes virtuous, since their only 100 stress ability is Rapturous.

Also try to get rid of the Weak Grip on Life quirk(in the Sanitarium) which subtracts 10%. Keep an eye out for the Unyielding quirk, which adds 10% and and lock it on to your characters in the Sanitarium. Tenacity Ring - Uncommon - A random drop from the Ruins +10% Martyr's Seal - Very Rare +12% Holy Orders - Very Rare - Crusader only +12% Eternity's Collar - Very Rare - Flagellant only +10% Dark Tambourine - Very Rare - Jester Only +12% Ancestor's Tentacle Idol - Ancestral +8% Stay away from trinkets that decrease DB resist: Gambler's Charm and Book of Holiness are both -10%. So in order to increase your odds of success without sacrificing lots of characters there are several trinkets(all can be purchased from the Nomad Wagon with the exception of the Ancestor's Tentacle Idol), a quirk, and a virtue that can increase your death blow resistance. A Flagellant increases the odds to 20.7%, which isn't that much better. Therefore the odds of getting this achievement with 5 resists in a fight for a normal character is only 13.5% so you may need to lose a lot of people if you want it to happen on its own. The base death blow resist for each character is 67%, with the exception of the Flagellant class which is 73%. When a character is at zero hp(Death's Door) any incoming damage from attacks or even a bleed/blight tick will trigger a death blow check.

Flagellant virtue darkest dungeon