Read what your trinkets actually do and don't just slap them on because you've got them. Remember to camp! Ideally you want to camp just before the boss for heals and buffs, ensuring that your team is in the best shape before they can be before facing the boss. Go in with a team that has some experience and their weapons, skills and armour upgraded, not a bunch of level 0 recruits straight off the coach. Note the Crusader has attacks which deal bonus damage against the unholy enemies which are common in the Ruins. A lot of enemies in the Ruins are highly resistant to bleed, so bring characters that can do either regular damage or blight. There are quite a few objects in the ruins that give a benefit if used with holy water, including large stress reduction and healing, so probably bring some of that too. Remember you can use food to restore some health outside fights, it's not just for camping and hunger checks.

Bring plenty of food, torches and shovels. which can add up over the course of the dungeon. Messing with most objects without using an item has a chance to cause stress, damage etc. Generally you shouldn't poke things unless you have the right tool for them. Note that boss spawns aren't entirely random they're usually in the room furthest from the entrance. This will mean less damage and stress from fights, traps etc. Until you get a bit better, try to get to the boss quickly instead of exploring every room. It's somewhat hard to give advice without knowing what sort of team you're running or what you're doing to get someone killed before reaching the boss, but some general tips: Having three actions per turn instead of four makes fights a lot harder, and if someone is dead your remaining characters probably aren't in great shape either. You probably shouldn't be fighting him if you've already lost someone.

Necromancer darkest dungeon