I realized that wasnt the right tail at the last second, 2 hits more and it would have been to late. In cases like this i could be useful to close the game and reload. You'll be in front of the fog gate, but remember estus are not replenished. And for the moonlight lure Seath to attack while near the crystal and quickly run behind you, you'll get a good window of time to severe its tail. Otherwise it can be tricky cause he keeps insta-turning on himself every time you try to reach that tail.