I was trying to work on it too and he threw me off lol. No Makers Fingers on the tail?
The tail is complete... nonsense (to use the polite term). His main method of knocking the Arisen off is to wave his tail around in solid rock, which lazy implementation allows the tail to go straight through while the Arisen collides with it and falls off. To make matters worse, they've designed him to actively abuse this ridiculous strategy.
I did finally destroy the heart just now, but I got knocked off about 8 times in the process. I spent over half the time running around after the end of the dragon's tail jumping at it and cursing at my television when the Arisen repeatedly failed to grab on (the grab detection in this battle seems to be the worst in the game).
I have little doubt the heart will be back in the next fight.