Edited for expedience. If you’re asking PvE, ignore this post. I know nothing about PvE.
PvP wise, I’m going to paste a response from another thread I think might be helpful.
First and foremost, your role is burst – there is no way around that. I’ve spent months trying to find some sort of survivable/hybrid spec (think like a Ranger using S/D and doing moderate, consistent damage), and it just doesn’t exist for thief. We don’t get protection or stability, our access to regen is subpar, our evade move (flanking strike) is a joke compared to how the Rangers works, and no immunity skill – without those boons, a consistent way to ignore damage (evades, blocks) and no immune skill, its not worth trying.
That said, you can do good burst damage without being a 2 shot wonder – this is what I’m currently running, and I like it.
http://gw2skills.net/editor/?fYAQNAoaVlUmaN3cS6E/5Ex+hey9gpGtp6MopNB;TsAg0CnoSykkIJbSukkJtsYYxECA
(might have to copy/paste into address bar to get the link to work).
Only 15 points into crit strikes might seem silly, but it works. Fluid strikes is kind a kittenty minor trait (how often are you over 6 init when you’re spending 6 or 9 init just to stealth), and while executioner is nice, you can offset it with other minor traits (Exposed weakness and hidden assassin). the loss of the major master trait kind of sucks, but it’s not spec breaking.
Shadowshot finally working correctly is pretty amazing – it’s 100% a superior gap closer to HS when your target is over 50%, and only does slightly less damage unless your target is under 25%, while also having double the range, not being affected by snare/root, and a blind attached. It also automatically does 10% more than the listed damage, as your stab arrives just as you blind them (with exposed weakness) if your target has no conditions.
Lotus poison and black powder give you a solid way to mitigate damage in melee while setting up your stealth combo (and increasing your damage 10% thanks to exposed weakness).
Devourers venom is awesome – now that poisons don’t vanish on missed swings (kind of old, I know, but I took a big break), venoms are a step closer to being useful. While a stunbreaker is Mandatory in competitive play, some classes don’t (or simply can’t) cram in a reliable way to break immob as well. 5 seconds of immob every 36 seconds will let you go nuts on a target if they can’t break it.
Also, if you’re not a fan of the venoms, you can switch em out, and replace Quick Venoms with Dagger training – thats 5% more damage all the time, and 10% more when your target has a condition (nearly all the time) – those 15 points moved from crit strikes to Deadly arts results in 15% more damage all the time, and all you lost was 20% more damage on targets under 50%, and 10% damage while over 6 init (again, not something you’ll see often in my experience).
Now for the spec’s shortcomings – without Fleet shadow, you have to run SoS, which eats up a utility slot. You’ll find it harder to get behind targets while stealthed with SoS than with fleet shadow, but all that lovely regen should help mitigate that.
I have a hard time finding a spot for shadow refuge in the spec, which hurts. Shadow refuge is a great tool in TPvP, and you will miss it at times.
You’ll find yourself a little initiative starved at, especially if you get Knocked out (or otherwise CC’d) of Black powder before you can stealth (you just spent 6 init to stealth and got nearly nothing for it).
All in all, I find it fairly effective – I still hit pretty hard, stealth on demand (IE you cant block/evade/immune my ability to go into stealth like you could with CnD), regen in stealth which allows you to absorb a bit of the random AoE you’ll be eating while positioning, and solid CC. It’s also worth noting that while the 300 toughness from shadow arts isn’t anything amazing, it’s alot better than running base toughness.
tl;dr – Try spec. Is good. Many reason.
