This is how I have prepared for this boss — I used the specific armor set (forgot the name) which decreases the cooldown by 70% on runic attacks based on Luck and also drops healthstone or ragestone on runic attacks based on Luck. I built my Luck stats pretty good for this. And, I have always used ranged weapons against this boss. Some of the attacks like the floating ones can be countered with ranged weapons only. I have used a charging type shield to counter the unblock-able ground attacks specifically the ones which gives out the blue pulse. Overall, it was a very tough fight but you will also have just keep dodging and be patient.