You would be surprised how much damage assasin dishes out. KE might be immortal when played right but assasin puts things down fast.
The assasin skill tree has great synergy with many other branches. Passives you want are: Dance of death, sneak attack, looked like it hurts, I was never here and knife in the shadows. Once you have all 5 you are doing the following things
1)automatically dealing a crit coming out of stealth
2)even out of stealth your crit chance is doubled as long as you are flanking
3)each crit or kill gives you stamina and removes cooldown on your stealth. If you kill your target, the cooldown is removed entirely.
You can make this even worse for your targets by picking up ambush and cheap shot, both of which ignore armor (ambush works on crits while cheat shot when coming out of stealth). If you get them as well your mark might as well be naked.
As for active abilities zeypher has the right idea, but he missed Shadow strike and it's upgrade, quick blade (both in subterfuge). This ability not only deals lots of damage but also knocks your target down. The quick blade reduces cooldown timer of all your abilities by 4 seconds. This makes for a great capper once you used all your other actives. Flank attack, stealth and hidden blades really shine when they are upgraded. In fact I would say you have to get them ASAP.
For gear I would say focus on DEX but also try to get your crit up to 40% or higher. Sneak attack will double that and make almost all your attacks crits, so cunning is important as well. Flanking damage bonus offers the most bang for the buck according to reddit, but I think getting your crit chance up is more important. I use daggers that give hidden blades 10% of the time since you attack so fast and armor that grants guard or increases your movement when stealthed.