The way I decided targeted attack works is, you lose advantage or gain disadvantage.
I say he has 4 rings, two on each hand, you have one shot to hit the hand, if hit you will potentially cut off the fingers in question. It's by no means a guaranteed maneuver.
If someone grappled and mobilizes him, then the successful hit will accomplish it without a second roll.
To lop off a head, you need a targeted critical hit or great RNG.