The standard Assassin approach will be to shadow walk and disappear from your sight, and hit you with a Swift Attack or rarely Dagger Throw. There is no way for you to avoid it unless you are lucky cause if they are good, they will hit you end of story. They will follow up with another Dagger Throw that can hit fallen enemies at level 3 or 4, can not recall, if you fail to recovery roll. And if after that hit you still fail to recovery roll, they will come in with a Cutdown and a Aerial Frenzy air combo, after which, they will use Ambush to relaunch you into the air, yet for another Aerial Frenzy, and if they can keep Tales Of Pirates money up, and there is no miss which allows you to fall back to the ground for a recovery roll, you will be trapped in an infinite loop, and die.
When you play against an Assassin, you will be the agressor, cause the Dagger Throw really hurts, and its a low CD skill, good sins will use it on every CD, and with each hit, you will have less chance of winning. Besides they get to score a free hit with Swift Attack every chance they get. So unless you make the move, you will only see your HP goes down and not his.