Silver weapons harm them. I've had a lot of luck catching them offguards while in Sneak mode and then hitting them with silver arrows for X3 damage. About four of these will do the job and with luck they'll never notice you. You can always finish them with a silver melee weapon or lightening blasts. I've also found that they don't really fly so much as float a specified distance above the floor so you can get out of their reach by jumping up on walls and things (unless you're in a cave
). When they go partially invisible just keep attacking them (you can see some wispy traces) and continue to do damage.