Like I said before, this attack is completely fine when he's using it from range
https://gfycat.com/GargantuanGlamorousIggypops
but the problem is if he uses it while you're under him (pretty much where you need to be to deal good damage to him and to bait out his tail swipe) there literally is not time to dodge it. If you go through the gif I posted on the other page frame by frame, you can see that in the exact frame where he drops his right knee (before he even starts charging) my health bar is already empty.
https://i.gyazo.com/ebe8180b950692ea9b77565662e829f4.png I agree that I should have managed my stamina better but that's not the issue here - I just don't think it's reasonable that a boss can instantly kill you for being positioned slightly to the side of where you should be for him to not do that one attack. I also didn't have any problems to this extent with fast attacks with any bloodborne bosses, however pthumerian descendant's sickle throw comes close.
(side note: the bandit knife is much better than the weapon I was using there, because swinging it costs so little stamina and with carthus rouge you can blitz through his health bar pretty quickly. I'm pretty sure the only reason I beat him at sl1 is because he didn't use that one attack while I was under him, but rather used one of the couple of other charging attacks where he gives you enough warning to get out of the way. Also you can mitigate the chances of him doing the undodgeable running attack if you stay slightly to the side of his leg instead of directly in front of it
https://www.youtube.com/watch?v=69XMmM0T5uU)