First time for me to make a guide, I do my best to keep it clean and understandable for everyone.
Feel free to tell me if I miss something or for grammar errors. I'm just a French guy who tries to help most people.
Enjoy.

I recommend you to do some Immortal run, you will keep your progress after defeating a boss, and if you die you will only lose one stat point.
The second thing to practice, it's the parry, in Nightmare difficulty it's you'r go to for surviving because you didn't take damage on parry + the stagger effect is very powerful. Be the parry god
Keep in mind that you will try multiple time in Nightmare and die a lot, especially at the beginning of each run.
Stay calm and pray the Sun.

Honestly play what you want, If your not very experimented I suggest you to have a strength character, or a ranged one like the scoundrel class.
Otherwise the class is up to you.

Here's a checklist:

• Ring the bell for move speed
  
• Taste the well for some red potion or white potion
  
• Check for the blizzard spell, it's for me the most powerful spell in the game, if not check support spells like heal, sanctuary or any boosting spells.....
  
• If the well have any colors said previously take an empty bottle, and at least one lockpick
  
• Take any potions heal, and cure physique/mind aliments or cure all, try to always have one of each in your inventory.

Well check list time:

• Always have an eye on the durability of your equipment, try to let the enemies break boxes for you. If the number is red it means that the next hit it will break, same thing whit your HP, if you have red HP numbers the next hit will kill you.
  
• Agro one enemy at the time (use range weapon spells or barrel), if you can't try to make them do a line (the doorway or gate is your friend for that, they will make them hit each other).
  
• Use your traps at your advantage, stagger them in spike trap or a row trap kick them in. It's all about preserving your durability and stay out of the danger zone.
  
• Clear as most possible, but don't risk if your low, it's a difficult choice because you need the exp and loot, it will depend mostly on your confidence
  
• Parry, parry, parry, all the enemies have the same timing: 1.2 parry .Don't be afraid to parry with the rock, you will not take damage on perfect parry. Only the stagger time is different with a weapon or shield.
  
• Look for the ring or power: recycle what's broken and chance to regain item. Thanks to this you will have a ton of trash and make powerful weapon or/and spells, in the trash combination list check this guide https://steamproxy.net/sharedfiles/filedetails/?id=2753575528
  
• Enchante your weapon and shield when you can.
  
• And last but not least play safe and don't be afraid to use your potions.

Just for you some other tip to keep in mind.

• Don't overpower your weapon damage because the durability loss scales with the damage you deal with it, doubly so if you hit a wall. Same thing whit armor durability loss, it's based on how much damage it actually reduces.
  
• Visual of parry
  Neutral
  
  Attack wind up
  
  Attack : You need to press the block button at the same time
  
  Parry : Not the right timing, you do not take full damage but the ennemy his not staggered
  
  Perfect parry : Concragrats you have done a perfect parry the ennemi his staggeur and you do not take damage
  
  
• Some combo that I really like and help a ton:
  Item/spell 1

  Item/spell 2

  Effect

  Recycle What's Broken

  Recover What's Spent

  You will have a ton of trash and have a chance to don't spend trash when combining them

  Blizzard

  Phantom Rush

  Blizzard will freeze them after every hit of phantom rush both dealing damage and keep them in place

  Transfusion

  Sanctuary

  Cast 2 or 3 Sanctuary then 2 transfusion you are at full mana and HP

  Ranged attacks

  Ranged weapon

  It cost double the durability, but you will have two projectiles, double damage !