To be honest you are better off asking this on the Ele forums, as they will be able to give you more in-depth advice. As far as I know D/D eles destroy thieves. But let's see:
- Backstabbed! What do I do when the thief hits me out of the blue?
Dodge the next CnD so he can't stealth again. Earth -> Earthquake. Or Mist Form if you are CC'd, most thieves will actually waste their entire initiative before realizing it's an invulnerable target, but let's not count on that.
- How can I beat a thief that keeps stealthing and running away whenever he's close to death?
Same way as everyone keeps you from running away with ride the lightning. CC.
- What do I do against a thief that "perma-stealths", that is, uses combos to remain in stealth for inordinate amounts of time?
Do what everyone does, use AoE and prepare for the inevitable moment when he will show up again. Static Shield is great here.
If all else fails, ride the lightning away and reset the fight on your terms.
Edited by Commander Boreale, 09 December 2012 - 04:30 AM.