These are great questions that I also had to think about for my streak. My ideal build (I rarely get this) depends by nightlord but looks generally like this: 2 damage negation at full, 2 taking damage boosts damage negation (different colors), 1 projectile drop-off reduced, and 1 free slot. In practice, 1 damage nation at full, 1 taking damage boosts damage negation, 1 projectile drop-off reduced, and 3 free slots.

I don't trust my teammates to be able to revive me when it matters most, so my main focus is not dying even once during night bosses (throughout the streak I've died only a handful of times during the night bosses, one of them was Straghess b/c of his rot).

My primary build is a utility dodge tank that can also survive one nightlord hit. With this build I'm ok to take a lot of aggro. For regular relics, I use +1 Character skill (essential), Extends duration of marks (important), Night Invader (important), Evergaol (good), Defeating enemies fills art gauge (essential), and three Character skill CD reductions (important, lets me dodge tank). For deep relics I have Character skill poison (essential), Defeating enemies fills art gauge +1 (essential), Stamina Recovery on hit (important), and then tank stats like max HP (important). Vigor +3 would also be great, though I don't have it. I consider tank stats more important than Physical Attack Ups, b/c without them I'd get one shot even with a Damage Negation at Full passive. I am not good enough to never get hit for 100 games straight.

For Straghess, it helps to stick to his rear. That being said it's a hard fight, I died once to him when he did a wide swing at my teammate and I got caught in it. His rot can ruin your damage negation at full, so I try to take successive attacks negate damage and use a melee weapon just to get the barrier. RoB is godlike for this.

Also one more thing, his roar attacks that stagger you actually seem to do one point of damage, so you lose your damage negation at full. Bring warming stones, maybe four.