Three quick habits: checkpoint your stamina, mark one heal window, and shrink your UI noise so you can see the hit that matters.
Lock in your stamina checkpoint

Every time I say, "I can squeeze one more hit," I get clipped, panic-roll twice, and die with my flask still lit up on the HUD. So I started using a dumb little checkpoint that turns stamina management into muscle memory: I never let the bar drop below one specific visual marker unless I'm committing to a punish that ends the phase. Pick something you can spot instantly in your UI. On a lot of games it's easy: the stamina bar has a notch, a color change, or a consistent chunk you recognize after ten minutes. Your checkpoint is "one roll and one defensive action." If you can't do both, you are not allowed to attack. Period.
Here's the practical version. Before you touch the fog wall (or the arena trigger), stand there and do the three-action test with your current loadout: swing your main attack twice, then roll, then try your guard/quickstep/parry. If the last input fails because you're dry, your checkpoint is too low for that weapon's rhythm. Either stop at a higher bar position, swap to a lighter setup, or accept that your punish string is shorter. I do this after every respec too, because the same boss suddenly feels "unfair" when really I just changed from a straight sword to a colossal weapon and kept pretending my old stamina habits would carry over.
What I like about this tip is it survives stress. When the boss goes into the scary part with the music swelling and your brain going blank, you don't have to calculate frames. You just glance, see you're below the checkpoint, and you back off. It's not glamorous, but it's the difference between taking the clean reset step and doing the tragic empty-roll in front of the entire arena.
Name one safe heal window, then defend it

I used to treat flasks like a fire extinguisher. Health dips, I slam the button, and then I watch the boss run a full cardio sprint across the arena to punish my sip. Once I started clearing bosses consistently, I realized I wasn't getting better at dodging. I was getting better at choosing one heal window and stubbornly saving it for that window.
Do this on purpose for your next boss: in the first 60 seconds, don't even try to win. You're scouting. You're looking for exactly one moment where the boss finishes an attack chain and has to do something that buys time: a roar, an element infusion, a leap that lands far, a recovery animation where they pose like they're in a cutscene, whatever. Your job is to label it in your head with a short name like "after the three-swipe" or "when it backs up to charge." Then you protect that window the way you'd protect your last estus in a marathon run.
That changes how you play immediately. Instead of healing at 40% because you're nervous, you start playing slightly safer until you can trigger your window. You'll disengage earlier. You'll stop chasing an extra hit at the end of a combo. You'll use terrain and lock-on discipline to make the boss do the pattern that grants your sip. If the game has a quick item wheel (Elden Ring's pouch, Monster Hunter's radial menu, anything like that), put your heal on the easiest access slot for this fight and put everything else somewhere annoying. You want one clean input, not a fumbled scroll that opens a throwing knife while you're trying to drink.
This is a Game Tips thing more than a "git good" thing because it's about building a repeatable routine you can run every pull. When you can say, "I heal after X," you stop treating healing like an apology for mistakes and start treating it like part of your rotation. The funny part is you often use fewer flasks even though you're healing at higher health, because you're not eating the extra punishment hit that comes with panic-sipping.
Turn down HUD clutter so you can read the tell

If you're stuck on a boss and you're watching guides, you've probably heard a lot about "learning tells." Cool. Here's the boring obstacle nobody mentions: half the time, the tell is on-screen, but your own UI is sitting on top of it. Damage numbers, status pop-ups, a giant quest tracker, party frames, buff icons, chat, streamer overlay, whatever. Bosses love to start their nastiest move during the exact moment you proc bleed, trigger three talisman effects, and light up the bottom of your screen like a slot machine.
So I do a specific pre-fight cleanup that takes two minutes and pays off immediately: I reduce visual noise until the boss's shoulders, hands, and weapon are always readable. In games with HUD opacity or "minimal HUD" toggles, drop it. If there are options for damage numbers, turn them off for the learning pulls. If you can scale UI elements, shrink the ones you don't need mid-combat. On PC, I also kill overlays I don't need (Steam FPS counter, Discord overlay) because they always end up parked right where I'm trying to read a wind-up.
Then I pick one cue and commit to it. Not "watch everything." One cue. Maybe it's the left elbow lifting, maybe it's the footwork, maybe it's the moment the weapon stops and hangs in the air for a beat. The point is you can't train your eyes if your screen is constantly shouting at you with icons. If the game has a photo mode pause or a replay theater, use it once: freeze the boss mid-wind-up and look at what part of the model changes first. It's usually not the obvious thing.
I like this tip because it's not about reflexes, and it doesn't require you to grind levels. It's just a settings pass plus a tiny bit of discipline. And yes, you'll miss seeing big crit numbers for a few attempts. You'll get over it the first time you dodge the move that was farming you for an hour because you could finally see the start of it.