Eventually I pivoted my build to a #Nagakiba with #Unsheathe and found the fight infinitely easier.
https://fextralife.com/elden-ring-intelligence-samurai-build-guide-soul-samurai/
Each attack did practically double damage. I had so many more punish windows with Unsheathe, rolling R1 attacks are actually viable, I even got an occasional stance break.
I'm not sure if Rellana's twin blade is just too stat hungry for a NG build or what, but I didn't feel the need to change build for *any* other DLC boss. The only similar experience I had was my first playthrough of the base game where I took a shield into #Malenia and watched her heal faster than I could do damage.
After switching build I beat him on like my third try. Not because I learned his phase 2 moveset but because I got lucky a couple times, had enough flasks and resistance to stay alive, and plain out-damaged him.
⚀ ⚄ ⚀ ⚂ ⚄→#cartridge
⚅ ⚁ ⚄ ⚅ ⚁→#trident
⚅ ⚃ ⚅ ⚃ ⚂→#untimely
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚁ ⚄ ⚁ ⚀ ⚄→#empathic
⚄ ⚁ ⚃ ⚁ ⚃→#roundup
cartridge-trident-untimely-unsheathe-empathic-roundup
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⚂ ⚀ ⚂ ⚄ ⚀→#footwear
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚄ ⚅ ⚀ ⚀ ⚄→#stability
⚁ ⚅ ⚃ ⚀ ⚄→#falcon
⚃ ⚂ ⚂ ⚄ ⚀→#oxidant
⚂ ⚁ ⚃ ⚁ ⚅→#goldsmith
footwear-unsheathe-stability-falcon-oxidant-goldsmith
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⚅ ⚄ ⚀ ⚃ ⚅→#unwired
⚃ ⚃ ⚃ ⚅ ⚁→#pleading
⚀ ⚁ ⚄ ⚃ ⚅→#autopilot
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚄ ⚂ ⚀ ⚂ ⚅→#sauciness
⚃ ⚁ ⚀ ⚅ ⚁→#neuter
unwired-pleading-autopilot-unsheathe-sauciness-neuter
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⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚀ ⚅ ⚄ ⚁ ⚃→#conduit
⚅ ⚁ ⚀ ⚅ ⚂→#think
⚃ ⚀ ⚅ ⚅ ⚂→#nature
⚂ ⚄ ⚂ ⚄ ⚄→#kilogram
⚃ ⚄ ⚂ ⚁ ⚂→#prevalent
unsheathe-conduit-think-nature-kilogram-prevalent
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⚅ ⚂ ⚁ ⚅ ⚃→#unaware
⚂ ⚄ ⚅ ⚅ ⚂→#legwarmer
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚄ ⚅ ⚄ ⚀ ⚀→#strangely
⚁ ⚁ ⚀ ⚄ ⚂→#dangling
⚃ ⚅ ⚃ ⚁ ⚅→#rash
unaware-legwarmer-unsheathe-strangely-dangling-rash
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⚅ ⚅ ⚅ ⚀ ⚀→#yelp
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚂ ⚅ ⚅ ⚄ ⚅→#mascot
⚂ ⚀ ⚄ ⚄ ⚄→#froth
⚂ ⚂ ⚄ ⚀ ⚀→#harmonica
⚃ ⚂ ⚁ ⚃ ⚄→#overpay
yelp-unsheathe-mascot-froth-harmonica-overpay
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⚄ ⚀ ⚂ ⚀ ⚀→#remark
⚁ ⚅ ⚀ ⚃ ⚁→#exclude
⚁ ⚃ ⚅ ⚅ ⚅→#elf
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚅ ⚄ ⚄ ⚃ ⚄→#vicinity
⚃ ⚅ ⚄ ⚁ ⚂→#reborn
remark-exclude-elf-unsheathe-vicinity-reborn
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⚀ ⚂ ⚃ ⚂ ⚀→#blemish
⚀ ⚅ ⚄ ⚂ ⚄→#confiding
⚁ ⚀ ⚄ ⚁ ⚀→#crumpled
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚄ ⚂ ⚂ ⚀ ⚃→#scorer
⚃ ⚂ ⚀ ⚂ ⚄→#overblown
blemish-confiding-crumpled-unsheathe-scorer-overblown
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⚀ ⚂ ⚃ ⚁ ⚀→#blatantly
⚁ ⚂ ⚅ ⚄ ⚅→#doorman
⚅ ⚃ ⚄ ⚂ ⚀→#unsheathe
⚃ ⚄ ⚅ ⚄ ⚃→#punisher
⚅ ⚃ ⚃ ⚀ ⚁→#unproven
⚄ ⚂ ⚀ ⚅ ⚀→#scalded
blatantly-doorman-unsheathe-punisher-unproven-scalded
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Doug Parker 🕸️
dicewarebot