in WvW I don't enjoy playing against *any* -/D thieves simply because they are so hard to maintain target on especially given the visual bug. That said I have the theory down on how to take them, just have to work on the practice.
All -/D thieves rely on Cloak and Dagger (CnD) to stealth them repeatedly. This means sometime during their rotation they have to run into melee with something and hit it to land stealth again. Since they have to do it so often, most -/D thieves are very good at landing and timing their CnD but it's still the most predictable attack they will use. It costs 5 initiative which is half or close to half of their total init at any given time, and even the most init intensive trait builds can't use it more than oh, 3 times in a 10 second span before their initiative is out. More commonly they have 2 swings before they have to wait for init recharge. This means the easiest way to counter such a thief in a true 1v1 situation is to prevent the CnD by blocking, evading, blinding or interrupting their attack run. Good long immobilizes do wonders since an immobile thief can't get into melee range, and most such thieves won't be able to cleanse conditions outside of stealth.
For guardian GS, Binding Blade is an excellent ability given you can land the debuff as they run in, and even if they land the C&D, you can follow with a pull, Mark, Whirling Wrath combo that will hammer most thieves out of their HP. Guardian hammer is also decent choice as a Ring of Warding timed correctly keeps the thief to a known location.
The really good thieves will ply their trade in a target-rich-environment specifically to avoid people who know how to dodge CnD, since they can unload their burst on you, then turn and CnD a poor hapless chicken or Pug nearby. The only real way to deal with that is to try to wipe all neutral mobs and critters in the area prior to engaging, or relocate to a spot less advantageous to the thief. Yes that means running away. Pugs and allied npcs you can only do so much
Target-Rich-environments significantly raise the difficulty and there are so far only a few solutions I have: Any character exiting stealth creates a small black cloudburst when stealth wears off. While stealth culling issues may leave the thief invisible for up to 2 more seconds, at this time they are actually tab-targetable. Using tab or "Target nearest" command can highlight a thief before he finishes rendering and allow you to lay down some punishment.
On a similar note, Pets, Illusions/phantasms, Spirit Weapons and other summons home and target stealth characters immediately as they exit stealth and are not subject to the visual culling effect. While they also provide more targets to CnD, sometimes it's worth it, especially high damaged ranged phantasms like Duelist and Warlock who will launch their attack as soon as the thief breaks stealth, or spider pets who can drop ranged immobilizes.
Lastly a suggestion that only applies to P/D thieves. These thieves are some of the most annoying since they do not use initiative for anything EXCEPT Cloak and dagger. The attack pattern is going to be stealth, position self, sneak attack, kite, vital shot, vital shot, CnD, repeat. As long as you have a class that has reflects and good reflexes, you can drop a reflect right as Sneak Attack launches, and tag the thief with his own medicine. Alternatively you can drop a WoR or (traited) Temporal Curtain, and step to the other side when the thief tries to attack. It's tricky but doable.
Other thief stealth skills:
Hide in Shadows (Heal): 30s Cooldown Treat as if Thief has used a CnD on another target, will destealth in 3-4 seconds or when they attack again
Blinding Powder: 40s Cooldown, 32 traited. Same as above.
Shadow Refuge: 60s Cooldown, 48 traited. This can, by itself, give the thief a full 12+ seconds of stealth. Usually used by thieves as a panic button getaway, or a reset, there are only two ways I know of how to handle this. The first:thief must remain in circle to gain stealth. That means you know WHERE he is. AoE, PBAoE or melee autoattack the heck out of the area. Thieves are squishy and if you land enough hits he'll go down, stealth or no. The second: SR has a "tripwire" effect, if the thief leaves the area he will automatically unstealth and get hit by the "Revealed" debuff. If you push/pull/launch a thief out of the circle, he'll go visible and be unable to use it. Most effectively proven by mesmers with Temporal Curtain/Into the Void or GS5 skill, Guardians with Binding blade, Shield of Absorption or a lucky Banish can also kick thieves out of their protective circle quite easily.
Edited by ProfGast, 08 January 2013 - 10:39 PM.