They don't seem to arc much... Mammoths have a ton of HP. I managed to take one down at lv 7 on Master Difficulty.

It took forever. I ended up climbing up a stair case in front of the fort outside of Whiterun to avoid it while firing spell and arrows at it. Then I got flanked by some bandits that came out of nowhere. I had to finish them and keep track of the mammoth. I frenzied one of the bandits and it went after the mammoth that quickly felled him.

Anyway, good match. All that was to say, mammoths have a ton of health and you might not be able to see the hp damage.