Condition Specter is a safe roaming build that has the luxury of also being able to duel a lot of classes, you could even call it a very mobile sidenoder.
Specter is slow and less stealthy compared to other thief variants. Infiltrator's Strike will help you move from fight to fight, but it won’t help if you try to go for a decap. Most of your mobility comes from wells and obviously Shadowstep, but since this is also your only stunbreak, use it with care.
This build is really straightforward when it comes to dealing damage. It all comes from Poisoned and Torment. Measured ShotEndless Night is your hardest-hitting ability. It has high base power damage and really good condition application, you want to spam it to get the most damage done possible. Siphon also brings good pressure, applying Slow and giving you boons, you want to use it off-cooldown but be careful to not miss it as it also provides you a lot of shadow force.
Another way to deal damage is to Enter Shadow Shroud, this gives you access to an entire new weapon set and a new healthbar. Grasping Shadows deals high damage, try and snipe people with it or CC them into it as the damage only procs a second after you place it.
Eternal Night also offers good damage but it’s mostly used for Chilled which applies a lot of pressure on a target, but be mindful of its very small range forcing you to be melee to hit. Sadly shroud auto-attack isn’t great so you don’t want to stay in shroud for damage too long.
Most of your sustain comes from Enter Shadow Shroud but to get to use it you need shadow force, which is gained by using initiative, Wells, and Siphon. Wells also bring you some healing and 4 second of Alacrity each.
Pistol Whip is very good for dueling, it gives you evade frames on top of a CC. This wont pressure your enemy nor heal you but you can gain a few seconds on a node with pulsing AoEs. It’s especially good when trying to hold the node against a mechanist who spammed mortar and elixir gun AoEs.
Another way to pressure a node is to use Prepare Seal Area on it. Not only does it block projectiles, but also any enemy trying to enter or leave the area will be CC’d. You can put it anywhere and activate it whenever you chose.
Stealth is not unique, but a very characteristic ability for Thief, as Specter, despite not running shortbow you can stealth your allies with good AoE blast it offers, a simple combo would be:
Shadowfall is your best bet to kill or clutch a teamfight as it pulses pull for it’s whole duration, making it impossible to escape unless stunbreaking and dodging.
Mind Shock will CC around you at the end of the cast so you can use mobility skill to make it proc on any target. Dawn's Repose is a great skill to do so as you can chain the fear and the stun.
Specter is the best roamer when it comes to dueling, no other roamer should be able to match you and 1v1 you. The one exception is Interrupt Mirage, as it deals very easily with your pressure and can interrupt all your wells because of how telegraphed they are.