It sucks yeah, but once you get a feel for when each twin does it you can pretty much dodge it, I was doing this consistently after about 10-15 minutes of doing the fight. It went along the lines of this "Ok XXX sister is nearing 15% hp, its about this time she usually does the attack, i better run out", run out, dodge attack, go back in and finish her off. rinse repeat. if you can't do this then ask for a stoneskin so atleast you don't get 1 shot, or get more than 4.5k hp ^^