The Remnant is really buggy and it has been for more than a year now. I reported the exact same bug you’re dealing with right now, on PC, in June 2020. As luck would have it, it’s one of my bug reports that went unacknowledged
The good news is that I think I’ve managed to find a workaround. Basically, you have to make sure that you always approach the pool from the front and center, as much as you can. If you block the Remnant’s attacks with the shield and you’re not facing it from front and center, it will most likely go out of skew.
The issue seems to be that the client and the server have different information about which way the Remnant is facing, so when the animation gets executed on the client, the Remnant is rendered either floating above the ground or buried beneath it. Since the game first does a hit check on the client and then on the server, it becomes pretty much impossible to hit the Remnant with melee weapons. If the client decides you missed it, it won’t send the attack to the server. If it decides you hit it, it will send the attack and the server will ignore it because you missed it there.
One more tip I can give you is to carry a bow and enough snake arrows (or some other arrows that stack a DOT effect). If the Remnant does go out of sync, you can still shoot it and, eventually, kill it.