Try killing it without using any firearms :). I thought you can't use anyt so i did it that way. Took maybe 15-20 hits to down that beast. Once you dodge the first few attacks he circels you for a while and then just starts running away - i cornered it at some hill and stabbed a few times. He did run away tho, so i chased after him for like 2 minutes only to find out i needed one more stab to kill it.