My tactics are:
Take unpredictable edge so the assassin can not block your fleeing.

Use only skills that will leave him dead or defense less, or whittle him down without you having to stay in combat for more than a couple of rounds before retreating to recover. Eg for an ap, sleep or cleave. Shaman can try dropping a plague, or curse, but probably break off and heal.

Get assassin to chase until timer wears off.

If assassin doesn't seen to be taking much damage, clear out fast. You will not kill him before he has messed you up.