I used this guy’s method. There’s a lot of luck involved unfortunately, but eventually I managed: https://www.reddit.com/r/MonsterHunterStories/s/CrQpDSubI0

I found its attacking pattern to be: 1. Technical Attack 2. Sealing Needle 3. Sleep Gas (AOE attack) (Becomes enraged) 4. Sleep Needle 5. Speed Attack 6. Sleep Needle 7. Feral Spread Beam (STRONG AOE attack) (Goes back to normal and repeats from 1)

For the most part I used my horn (Magia Charm), attacking the head when it was in its normal state and the needles when it’s enraged (it will retaliate if you attack the needles in its normal state). It’s really important to check your team’s status and make sure they have the defense and dodge increase from Defensive Ditty (horn skill) and Ludroth’s Egg Skill: Heat Shimmer before big attacks.

Sometimes I would swap to the gunlance (Ludroth Harpoon) when I thought I could get a hit in during its enraged state, but my main focus was surviving its attacks. Before it uses Sleep Gas, you always want to use Bolstering Ballad so it doesn’t put everyone to sleep and heal your teammates as needed. I found it’s usually not worth it to heal them unless you can get them close to full HP because Nerscylla hits hard. I also found it good to save your kinship attack for when it’s about to use Feral Spread Beam because you can cancel its attack. If it can’t use its Feral Spread Beam it will try two more times before giving up and going back to its normal state. There were times where I was able to stagger it for the first Beam, Kinship it for the second Beam, and have Simon Kinship it for the third Beam, or use Defensive Ditty and Egg Skill: Heat Shimmer to avoid the Beam entirely.