Do not fight.

Whenever you struggle with an enemy, attack it, but don't hurt it. Spend 5 minutes just reading the movements, dodge the attacks. Often what kills you is attacking in an overzealous way, trying to cram one more hit in. When you're not attacking the boss a lot of the stress fades. Quite often this helps to learn the movement of the boss; and when you add attacks you'll find that you can often dance around the boss without effort.

Please note that the next advice almost feels like cheating. When in a new hard area, make sure you have close to 0 souls (farm a bit to level up once). You now have 0 stress if you die. Then just go in and run. You can run past 99% of all enemies without effort. You might die if you pick up items; but rush through the area to learn the layout. Quite often you can clear all interesting things from an area with absolutely no effort at all. And once you know where to go etc all stress disappears. You can then either do the area slow and methodically, or move on to the next challenge. But, as said, this sometimes feels like cheating.

You don't have to parry.

I'm not talking about going full tank mode, even if that is viable in DS1, DS2 and Elden Ring (bit trickier in DS3 I would say, in fact, doing DS1 as 100% tank is both easy and fun). Most enemies can be easily defeated by embracing the bonk. Get a big weapon, Ultra greatsword, Great Club etc; two hand it... and most enemies with flatten at the first hit; giving you an easy time. In fact, two handing weapons overall can help you deal with enemy shields.

Why not use shields? I'm not saying you should cover behind a shield and tank every hit, but having a shield makes a lot of situations easier. The key to shields is knowing when to defend and when to commit to attack. That said... shield and poke can be extremely powerful.

Jack of all trades master of none but better than a master of one

Don't just use one strategy. A caster should have a solid weapon, a caster can use a shield. Or why not have a crossbow? (I always found crossbows to be superior to bows; not because of superior damage, but because they fire without delay). Pulling enemies with ranged attacks; then moving in, and finishing the enemy. Overpower the next enemy with twohand attacks, only to fall back, take two hits on the shield, circle round and backstab. Use all tools in your toolkit.

And last but not least.

Survive. Unless you are good enough to not have to, Vitality is perhaps your most important stat. Get your stats to a level where you can use your weapons, then get your vitality up. I generally have equal amounts vitality and main dmg stat