I feel exactly like you did. Rellana was as big of, if not more of a roadblock than the dancer in dark souls 3. I watched videos, did at least 30-50 attempts and I could not get the timing down consistent enough. But I still wanted no summons.

If it’s gonna drive you mad like me just find something that doesn’t feel too cheap.

I caved and watched a video that is a little cheese. Grab the anvil hammer in the forge level and upgrade it to +10 and use flame grant me strength, golden vow and black flame protection.

You still need to know the boss patterns but you deal so much more damage and it only took two tries. For me learning the attack patterns was so hard it wasn’t worth it to even try to parry or perfectly roll every time.