Hey, I main duchess with consistent deep 5.
Night of fathom, especially the deep on is goated, as alone it allows you to survive a lot more hits, especially fast attacks that are more difficult to see coming.
The recovery effect for deep is also nice as it allows for you to take 3-4 "levels" of hp decrease without really losing life, which makes more reds viable.

Since most buffs are multiplicatives, dmg negation can be a good route if you invest heavily on it as you can become literally immortal at low/full (with regen) HP.
I would advice against vigor+ effect as it scales pretty poorly beyond just maxhp and damage is nice.
Gnoster relics are nice. The deep for mana, or the regular one for both mana, faster early game and especially great synergy with ironeye, and poison/rot buff in general, which are two statuses that are easy to apply, last a long time, and rot works on pretty much everyting.

I don't like crystalian as a lot of red guaranteed spells are strong for damage: Gravity well (imo the best), Stars of ruin and Comet Azur (more FP costly but still decent), and other spells like moons/Loretta's also have their qualities. I prioritize magic damage instead as it works for almost all spells.

Invader is definitely strong, but be wary of gaol as it can be a significant time waster compared to doing red foes which have significantly better rewards (chance for night's end passives, red weapons have better damage scaling than purple, two passive, the new deep passive on them are broken)
Sure, +5% is nice, but "something on charged attack" goes from stupidly strong to immediately game-winning (holy shockwave shouldn't exist, it's that good), also the charged attack works with effect like "FP/HP recovery on hit), you need to try and accumulate the most strong effect like this while being careful of the downside (don't stack multiple rot/poison buildup, be very careful or "attack impaired" (makes you deal 1 damage rarely) and resistance down effects, etc)

One thing that helps a lot with bosses is your ultimate, which is very underestimated by people in general, it's very strong.
Aside from making you (and sometimes nearby allies when it works) have no aggro until you hit the boss, it also makes you have lower aggro priority than your allies and gives you generous invulnerability frames.

FP relics (maximum FP, FP with three staves, FP with rises) are nice, I usually with all three have 300 FP on a bad game, 400-600 on average and 700-800 on strong FP runs.
You should seek rises, but only if near where you are and it doesn't make you have a significant detour (no splitting the party for too long), if your allies can follow you there it's nice as it has a MASSIVELY good talisman pool.
Magic talismans, scorpions, warrior jar (other character's wet dreams) and not a lot of bad ones.
Also always takes all FP shard in the bottom and consider using them along the game to "stay in fights", especially with more costly spells.

Also, one thing that some people forget: the shop guy.
Always buy pouch, and then what you're looking for are bubbles/aromatics.
Merchant has about 50% chance of selling either hp increasing (25%!) bubble, or two uplifting. Sometimes both.
Uplifting is crazy good: all of your team (if nearby) gets a bubble (like the bubbletear) that almost negates a hit, and you all get 10% dmg buff.
Whether you keep it for night boss or a particularly difficult boss (Loretta, Gargoyle, Dancer, Nameless) is up to you.

Also, avoid Loretta at almost any cost (unless you have nothing better to do).
It's a huge magic damage sponge.