I'll start this off with another disclaimer; I am only level 36, and cannot pretend to have faced every situation that Hyboria has to throw at me. I can, however, give new commers to the class some tips that have helped me thus far.
I'm going to stay away from builds and feats at the moment, as that's a different discussion entirely.
"Assassins are weak! We can't even take 2 mobs at the same time! I WANT MY MOMMY!"
Okay, say what you will about the 'Sin class, but we are definately not weak in the PvE arena. As much as I'd love to say, "If you can't take on two mobs, you fail. L2P or GTFO, noob!" I won't...because that'd just be mean. And hey, there was a time when I couldn't take on two mobs at once...though I eventually found out it was an...um...opperator error.
And occasionally I'll get mowed down by two mobs still...usually as a result of an unlucky chain of crits against me, or my royally messing up my combos. It's weird, but we have very little room for error. Either you execute perfectly and mow through the mobs like they were...something that gets easily mowed...or we miss-execute and die a horrible, embarassing death.
Tip one: Stance--it's not just to attract the chicks (Or dudes, if that's what you're into).
Above I mentioned that there was a point when I was mad because if there was more than one mob, I'd never get the second one killed before I died. Well, I was more than mad. I was ready to quit the class. Quit the game! Screw you Funcom! BALANCE!! ....then, as I was looking through my abilities book for something I gained upon leveling, I noticed "Poison stance" and "Unholy stance". What the hell? These aren't on my hotbars. Where'd they come from?
Well, to this day I can't tell you at what point you get the stances, or if you start with them. I was level 12ish when I discovered them (So I'm a bit slow, I can't help it). Well, I popped unholy stance 'caus, well, it's unholy...how cool is that? (Tired of Assassin = poison, too cliche for me). And guess what? My damage more than doubled.
Moral of the story--without a stance, you're basically a squishy weakling duel-weilding what might as well be plastic sporks from the school cafeteria (they do still use sporks in schools, right?). I don't care what stance you use--I prefer unholy, but the majority said poison is better. Now I think the community is split about 50/50. Again, I'm not going to talk about feats and builds, but keep in mind that just because you go corruption spec doesn't mean you always have to use unholy stance, and vise-versa. Certian talents won't work if you're not in the stance, but that doesn't mean you can't do it.
From the scant literature and "amazingly indepth" (puke) tool-tips, Unholy stance seems to be aimed at higher-armored targets (Part of the damage is delt as unholy damage, wich is not mitigated by armor, but by resistances), and poison stance is more for caster-classes (poison is, I believe, suppose to slow down their casting time). I haven't used poison, so have not compaired the two side-by side. That's the theory between the two, but in practice I don't know how the numbers add up.
Tip-Two: Combo's and shields (and bears, oh my!)
Obviously you want to use combos. For those new to the class, you can almost completely ignore the combo descriptions. They're awfull. For the first part of your leveling experience, Cunning Strike is going to be your big-damage dealer. The tip reads as if it's just a threat-mitigation move for when you're in groups, but it's by far your best combo. Swift strikes is also up there. But both of those are "Center combos"...in other words, both attack to the center shield zone. So if all three shields are moved to guard center (most mobs do this after your first cunning strike), then they become your worst combos.
So you'll need the other combos for when you can't attack center. My rule of thumb is, I do not attack any zone that has two or more sheilds, and I attack any zone first that doesn't have any shields (pending availability of the combo, of course). Think about it; a combo that does (for the sake of ease) 100% of 100 damage is better than a combo that does 10% of 300 damage. Aim for the unguarded spots!
I'll list the ablities that I use for each zone--though it's probably fairly similar for everyone as we don't get that many to chose from anyway:
Left side-Grim Corruption and Lethargy - at lower levels I was using GC under the idea that it lowers their unholy resistance, so subsequent attacks will do more damage. Which is true, but we kill mobs in two or three combos max regardless, so is not a deal maker. For a while I switched to lethargy, because (without close examination) it seemed to do more damage. I don't know if it does or doesn't, but now I'm back to using GC as my primary left-side attack because it's a single-button combo. Lethargy is still used as my secondary left-side attack, if needed (or against bosses).
Center- Cunning Strikes and Swift Strikes - Cunning strikes is my primary attack, but swift strikes is catching up to it (Cunning strike hasn't been upgraded since like level 20, Swift strikes gets upgraded circa level 32). Both do *almost* the same damage for me, with CS edging SS out by a small margin. Also (Important!) different ranks of the same combo are on seperate timers. So I can keep my next-highest rank of CS and SS as alternatives, for when the first two are both on CD.
Right side- Slow Death Strike and (Some ability I rarely use that gives a small bleed effect. Heh, sorry). The second is a back-up, incase I absolutely need to attack the right side and SDS is on CD. SDS does nice damage and then tics more damage for 3 seconds. Not a long DOT, but enough to make a difference, particularly on targets with low health.
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Since I mention "Sheilds" in the tip name, I'll briefly talk about your own dynamic shields. LEAVE THEM ALONE. :) If you're fighting something 1v1, and you have the ability to (I don't, I'm a little slow, remember?) you can adjust your shields to the zone he is hitting. If there's more than 1 enemy, which is normal for PvE, I suggest leaving them alone. If you adjust your sheilds for one, you are leaving one zone completely ungarded for the other to hit. Slightly mitigating one damage to greatly increase another is not worth it.
PvP can be a different story, but I'll talk about that in the PvP section.
Tip Three: Auto target
Okay, a lot of people say to turn auto targeting off. My response was "I'm an uncoordinated clod, I can't do all that by myself!". I was particularly worried about the PvP ramifications, but in short, it didn't hurt my PvP preformance at all. Recently, against my personal doubts, I turned auto targeting off, and tried aiming my attacks to land between two (or more) mobs. And guess what? It actually works. Because I can only see the sheilds of the Mob I have selected, and because I base my attack sequence on those sheilds, I usually don't kill both mobs at the same time. But if I'm hitting both, the second will be at least 50% dead at the time of killing the first. Which speeds up finishing the second. And if there's a third, then I typically have decent health left to take him out even if he's still at full health. Then the adds come along, and you take them out. Then you unleash a primal scream as you gut soldier after solder ftw! AAAAAAARGH! ...okay so it's not that dramatic, but it does substancially increase your odds on 1v2+ fights.
Tip Four: xBow 360, live(tm) ...don't worry, this is the last tip...For now >:)
I mentioned it in the stealth tips entry, but don't be afraid to use your xbow to pull in crowded situations. A lot of us get this "I'm a rogue (wrong you're an assassin) I need to kill from the shadows! (wrong, you need do kill with any means necessary). Don't let your predefined notions of an Assassin class gimp your play...it may not be exactly what you were expecting, but be flexible. I garuntee mowing down mobs with your daggers and slashing their throats feels very assassiny, regardless of how you gained agro.