A f t e r the tail is off, go on with mount spam. When he falls down again, aim his head. Then he stands up. At this moment put 2 Large Barrel Bombs (eat for Pyro) under his head and just kick them. The job should be done→end via subquest. Sorry, I can't put it in spoiler because it's a saved screenshot. Please don't report it. Yes, I got 3 in one run on my first try.