You can use your skill-w-r combo, and concentrate on keeping your HP and mana both high, you are going to be in for a long fight against a closely geared assassin. Most sins can combo to a good standard, so you must use your healing skills to put him under pressure. If your HP does not budge after his full combo, he will panic and try to combo faster, diverting his attention from his healing. It is amazing now, when you fight some sins that you used to think were strong, their combo is very fast, but they cannot heal under pressure at all, which makes them easy prey. They know who they are.
If you are not making progress, be patient and keep concentrating on your healing. On the other hand, your opponent will be neglecting his healing more and more as the fight progresses. Eventually he will make a mistake, and he will be forced to run away. You should stay close to him and do not let up the pressure. You should keep him illusionaed if he is far stronger than you in level and items.