Man is it a pain in the ass to cut off Kalameet's tail. I can basically fight that battle indefinitely, but the window to hit his tail is just way too small. It also doesn't seem to count when you hit the upper part of his tail, too. I probably spent 45 minutes trying to get the job done last night. I can borderline kill him without getting hit, but his stupid tail is driving me nuts. Videos show people doing it, but I swear it seems like they have more time to do it than I have.
my conclusion was that something changed in the timing when be sits to blow fire. I never succeeded to get into position to cut on his tail whereas I was able to in the original game. What eventually worked for me is to hang in front and wait for his dive. Roll right and again. It puts you under his tail. If he looks back then he will do a slam, otherwise move away because of a spin. Use the rolling ring since it is good in general for the fight.
And yeah, "technically" none of this is necessary for achievements, but it is satisfying regardless