I just get locked (targeted) onto the tenticles and hold down my draw arrow button to "charge it up". While I am targeted in on the tenticles, I just kept moving to the right ( or left, which ever you perfer) to dodge the tenticles from attacking you. After you kill one tenticle, target in on another. When they are all gone, just the head will remain.
If you look on the top of the tenticles, you will see white circles, if you hit that, it dies. If you have the skorm bow, it only takes maybe two arrows.
Edit: you hit the head just like any other enemy, but you must take out the tenticles first.