Openers vs each class.
Warriors, they always try to fear you first, you can garrote them as an opener, build up 5 points and 6 second kidney shot them. Bad warriors will try to bladestorm for dps, good warriors will try to bladestorm for mobility, point is if its 1v1 don't bother disarming bladestorm and save it for reck if they ever get to use it. Just Feint during bladestorm and you're taking 50% less damage from it. You could also evasion since they won't be able to spam those overpowers on you.
Hunters, Camouflage/Flare, Run in with sprint and fan of the knives, What experienced hunters will do is they will either Scatter shot, or Monkey Blind you. You can be gutsy and try to use evasion for 25% chance for them to miss scatter or you can disarm them before they do so, which is easier than it sounds. Monkey blind is Cloak-able. Also, 90% of the time hunters jump up before disengage to make the distance larger, when you see them jump and white sparkly light appear on them it means disengage was used, not u got less than a second to react but with practice it becomes easier. You can A, gouge it and they won't move, or kidney shot it, cancelling it too. Or you may disarm and simply run straight on at them since they can't conc shot you. One more, if you don't have any more vanishes but still have blind and you know the hunter doesn't have trinket, don't bother using it on the hunter since you can't re stealth with the pet on you. Instead Blind the pet if it still has Roar of sacrifice than Shadow dance and quickly cheap shot the hunter. Making him take the full damage.
Mages, If you have shadow dance you can use the Cast sequence macro from above to force a block from a frost mage or a trinket from a fire mage. If you do not, you can sap the mage, premeditation, than stand on an angle to them, meaning you are facing west and mage is north. Garrote, they will still be in sap before it ticks bleeds, than quickly kidney shot. If done properly you won't auto attack to break the sap right off the bat, and will be able to keep them nice and close to you for a 4 sec kidney shot eating backstabs like a boss unless they trinket ^.- OR block.
Warlocks, Only real tip I can give you is, open with garrote, don't bother sapping pet because of soul link, and do NOT shadow step before they use portal. it is a good idea to shiv the pet after you spend combo points because you may have enough time to blind and re stealth.
Priests, key rule is that priests are scared to disperse before shadow dance, well that's really good for you, because YOU don't need shadow dance to burst them apart with backstabs. I recommend an add-on that tracks enemy cooldowns and when you see fear is about to come off CD run out of their range and quickly back in, once in awhile you get to dodge a fear and that could be a game. Also, don't bother trinketing the first fear, they can't burst you yet. Pre-emptive cloak for fear is a gutsy move but can really pay off.
Paladins, never sap a retribution paladin. It gives them holy power, and why would you want to make it easier for them to heal your opener once its over? Gouge does the same thing but is situational. Open with garrote and follow up the usual. When you see them blow wings, vanish > Cloak IN THAT ORDER, and wait it out. Yes they will heal up most likely, but from than the fight becomes much easier.
Death Knights, Let's be honest, you eat death knights for breakfast. Yes you eat dead things for breakfast. Only problem could be blood. I usually open with garrote, quickly get 5 combo points, vanish full recuperate Than open with cheapshot > Kidney. If they icebound, prep vanish cloak if you need to drop diseases. wait it out, repeat. Make sure you have an add-on for diminishing returns.
Rogues, good luck with this one, may the better one win. But If I see you start the duel in stealth I will kick your behind to Pluto. Usually both rogue start by trying to gouge, from than it just carries on randomly.
DRUID, first feral, if the druid is just sitting in bear form, don't be afraid to put up expose armor, full slice and dice, and even rupture, they can't kill you like this, but its hard for you to burst them. once you see them go into kitty form get that 5 point Kidney shot, and with those things still ticking even bark skin won't save them. On bears open with garrote. UNLESS they have nature's grasp on, than Cheapshot > Kidney Shot to last it.
Boomkins, believe it or not a good boomkin can pose troubles, respect to Thehurricane from Sargeras.
When you see Nature's grasp, put them in a full Cheap shot > Kidney shot, if they do not use it right off the bat before you open, than open with garrote. They will keep faerie fire on you, and you should only cloak vanish when you're 100% sure you can catch them in caster form. Usually when they run and try to HoT up.
Best idea is to do it after you gouge ^.-.