Actually a good assassin can still backstab a good warrior, but it is also a lot harder. If you have good Combat sense (know when they strike) An assassin has a good chance at making them miss if they do miss, they will cause the enemy Warrior to lunge with the stance (or quick attack) Causing them to become..... very vulnerable Also, if you are cloaked they can not lock on to you (but they can if they locked onto you before you cloaked)
Use that for your advantage, and also if your high level Poison and Arterial strike is a very good friend for you. Assassins can basically out run warriors on Colosseum (jumping over the stairs etc) Especially with there low stamina regain