Gamer has been removed
Most guides recommend dodging her swings to attack her tail. This can be VERY tricky, given the quirky nature of the dodge button in this game. (Anyone else have a problem with hitting the Dodge button 2-3 times with NOTHING happening?) If she corners you, or if the camera rotates behind you, you're probably going to take a beating and die. I learned a few tricks that actually make this a relative breeze.
Equip a good melee weapon...the axe or the dagger. After a few moments, when she finally rushes you, DON'T ATTACK. Let her grab you and raise you up. Hit the correct button to shake free. When she drops you, you'll be right in front of her, and she will be momentarily still. Lock on, and do a quick attack. You'll have time for one or two "A" attacks, or one strong attack ("X"). (Sometimes I've managed a quick A-A-X combo). She'll do a tail sweep and knock you back. The tail sweep doesn't do a lot of damage (on Normal).
Get up and rush her. She'll usually grab you again (sometimes she won't, which I will get to in a second). Again, shake free, and repeat the above attack(s). Now, there will be times she won't grab you when you rush her. That means she going to use her vile breath on you, which causes significant damage and is usually followed by a "body slam," where she'll crash onto your chest, adding to the damage. To avoid that: When she fails to grab you, ROLL TO THE SIDE IMMEDIATELY. You should be able to dodge the breath attack.
Keep repeating the above 10-15 times or so (on Normal; the dagger allows faster attacks but takes longer to finish her. I usually use the axe or crowbar). She'll eventually fall back, and you can finish her with an "X" command. I've faced this boss four times so far (going for each ending achievement), and this technique has worked well each time. I typically use 2-3 health drinks.
I will be honest, though: Sometimes she won't grab you and will simply beat you into a corner and melee you to death. It's happened to me a couple of times. I just let her kill me, then reload the save (right outside the door, thankfully) and try again. It's never taken more than two tries to get the above solution to work, and it usually works the first time, so I feel confident that it will work for someone else. But it might take a couple of tries. Again, I believe in being honest.