Stop dying to your own thumbs: I rehearse three inputs in the menu, then walk through the fog gate calm.
Do a thumb warm-up on the options screen

Boss runs have this goofy little trap: you spend two minutes jogging back, your hands go cold, and then you ask your thumb to hit a tight dodge window on the very first swing. If I'm about to push attempts, I pause at the last safe spot (bonfire, grace, shrine, whatever your game calls it) and flip into the options menu for 20 seconds. Not to change settings. To rehearse the three buttons that always betray me when I'm tense: my dodge, my heal, and my lock-on.
Here's what I do. I rest my thumb on dodge and tap it in a steady count, not fast. You can hear it: click-click-click, evenly spaced. Then I move to heal and do the same, but I watch my grip. If I'm death-gripping the controller, my left hand starts steering without me and I get that micro-drift that ruins spacing. Last, I practice lock-on with the lightest press I can manage. A lot of us mash that stick like it owes us money, and then we wonder why our camera does something stupid right as the boss jumps.
This is Play Guides stuff in the most literal way: it's not a walkthrough, it's a pre-fight routine. If you're on PS5 or Xbox, do it with the controller you actually play on, not your backup. Different stick tension changes everything. If you're on PC with a DualSense or an Elite, same deal. You don't need a metronome, and you don't need a lecture. You just need your hands to remember the feel of those three presses before the music kicks in and your brain tries to drive.
Turn one attempt into a "data run" with two rules

Every boss has that moment where your brain lies to you. The arena looks huge, so you sprint in. The boss looks slow, so you swing early. Then you eat a two-hit combo and spend the rest of the attempt panic-healing behind a pillar you can't see past. My fix is a deliberately boring first attempt I call a data run, and it has only two rules.
Rule 1: no damage unless it's free. If I'm not 100% sure the opening is safe, I don't take it. I circle, I backstep, I let the boss show me the distance on its lunge. I'm watching feet, not health bars. You can learn more from ten clean dodges than from landing two greedy hits.
Rule 2: spend your resources like they're test equipment. I limit myself to one heal, total, even if I get clipped. That forces me to pay attention to what hit me and why. Was I locked on and got dragged into a camera spin? Did I roll late because I was mid-swing? Did the boss have a delayed follow-up I didn't respect? If I'm on a build with a parry, I don't parry yet. If I'm on a build with summons, I don't summon yet. I want the boss's baseline behavior, not the "summon chaos" version.
The Play Guides angle is the point: you're practicing how you practice. After the data run, I pause for five seconds and say out loud one sentence, like: "The slam has a second shockwave" or "The grab tracks if I roll backward." That's it. No notebook, no full breakdown, no big brain. Then the next attempt isn't a coin flip. It's a controlled test of that one sentence, and you can feel your inputs settle down because you finally know what you're looking for.
Fix panic camera with a one-minute right-stick reset

If you watch your own deaths (clips help, but you can feel it in the moment), a lot of "I got one-shot" moments are really "my camera went somewhere dumb and I rolled into the thing." This is especially true in tight arenas, huge enemies, and anything with a jump-over-your-head move. The Play Guides move here is to drill the camera, not the boss. One minute. No excuses.
Stand in a safe area near the boss door and pick a landmark: a torch on the wall, a doorway, a statue. Now do this sequence ten times: lock on, unlock, center the camera behind your character with a gentle right-stick tap, then stop touching the stick. That's the whole point, stopping. Most of us keep stirring the camera like soup, which means we're still inputting when the fight starts and the game is trying to help with its own auto-center. You get a little tug-of-war and suddenly you're staring at the boss's armpit.
Next, practice one clean 90-degree turn without touching the right stick at all. Left stick only. Let the camera trail and re-center naturally. If your game has camera settings, this is when you check them: turn off camera auto-wall correction if it's too aggressive, or lower sensitivity one notch if you keep overshooting. I'm not telling you to reinvent your controls. I'm telling you to make your camera predictable under stress.
The firsthand giveaway that it's working is how quiet your right thumb gets. When I do this, I walk into the arena and my thumb is suddenly free to do its real job: dodge timing. The boss didn't change. My view did. And yeah, it's boring. That's why it works.