A loot-ready pouch, a 10-second debuff sweep, and a quick VOD bookmark stopped my repeat deaths.
Build a pre-pull UI profile you can swap back to

I used to treat UI tweaks like a one-time thing. Then I noticed a pattern: after an hour of wipes, I'd start chasing small adjustments (move the boss frame, resize the cast bar, nudge the minimap) and I'd never quite get back to a clean baseline. The fix is boring, which is why it works. Make a dedicated "Boss" UI layout and save it as a profile you can swap to in two clicks, then promise yourself you won't edit it mid-session.
What goes into that profile depends on the game, but the Game Tips mindset is the same: surface only the information that changes your next decision. On my setup, I make three things obnoxiously readable: the boss cast bar (big, centered), my debuff row (larger icons, show seconds), and my resource (stamina/mana/energy) right under my character. Everything else gets pushed to the edges or hidden. If the game supports it, I turn on "always show" for dangerous debuffs and toggle nameplates to show only enemies, not friendly clutter. In WoW, this is where I'd set a raid/boss profile and lock it; in Elden Ring I can't mod the HUD the same way, but I can still standardize camera sensitivity, lock-on behavior, and HUD elements so I'm not fighting menus and muscle memory at the fog gate.
The practical bit: take a screenshot of the finished layout and keep it in the same folder as your keybind notes. When you inevitably drift and start moving stuff around, you can snap back to the screenshot and restore the profile. It's a tiny thing, but it stops that slow creep where you "improve" the UI into something you can't parse when the screen fills with effects.
The 10-second debuff sweep before you waste a pull

Here's a move that feels almost too small to matter: right before you pull, do a fast, literal sweep of your status bar and fix anything ugly. Not "yeah yeah I'm flasked". I mean your eyes go left to right and you say (out loud, if you need it) "food, flask, weapon, defensive, debuff." Then you click the one thing that's wrong and you start the fight clean.
I learned this the hard way in raid nights where we were convinced the boss was "spiky" when really half the group had a lingering healing reduction from trash, a broken weapon, or a missing weapon buff after a res. On action games, the equivalent is subtler: you walked in with a weird talisman swap, you forgot to re-equip your bleed cleanse, your armor load tipped you into heavy roll, or you spawned in with the wrong quick item selected. Those are the kinds of mistakes that don't show up as a big red warning. They just make the first 30 seconds feel cursed.
My rule is 10 seconds, max. If it takes longer than that, I'm messing with builds instead of practicing the fight. In WoW terms, I want to see: repair status good, weapon enchant present, the right consumables ticking, and no purple "you picked this up somewhere" debuff sitting under my frames. In Elden Ring terms: check equip load percentage, quick pouch has the one "oh no" item (like Preserving Boluses), and the Physick is the right mix. The point isn't perfection, it's consistency. When you wipe, you want to be debugging the boss, not debugging whether you accidentally queued the fight with a handicap you gave yourself in the hallway.
Bookmark your best attempt while it's still fresh

If you record your attempts (and if you're serious about a boss, you should, even if it's just local), don't wait until after the session to review. Do it in the moment, but keep it tight: you only need to bookmark one clip, and only when you get a noticeably cleaner phase than usual. The trick is to capture the "why" while your hands still remember it.
My setup is simple. I run OBS with a replay buffer so I'm not managing files between pulls. When a pull goes well, I hit the save hotkey during the run, not after. Then, while everyone rebuffs or while I'm walking back to the fog gate, I open that clip and scrub to the 8 to 12 seconds before the clean moment. Not the death. The clean moment. The dodge that landed, the heal window I didn't get punished for, the corner I used to make a projectile whiff. I pause it on the exact frame where the boss starts the tell and I look at my positioning. Was I on the boss's left hip? Was I near a pillar? Was my camera high enough that I could see the shoulder twitch? Those are reproducible details.
Then I write one line in a notes app: "Phase 2, stand on outer ring at 10 o'clock, wait for second swipe, then heal." It's not a paragraph and it's not motivational. It's a bookmark you can run. On the next pull, I glance at that one line before I step in. It keeps me from drifting back into sloppy habits just because the last three attempts were messy. And when the night ends, I already have the clip that matters, not 47 minutes of "we almost had it" footage I'll never watch.