I am not an assassin but I am a bh healer and in my opinion you should never be able to kill a healer 1v1 (assuming equal gear and skill), especially as a tank. What good would healers be at healing if they can't outheal one dps or tank(lol). The whole idea behind healers is to keep others alive, sometimes a few people at a time. So if you are the only one beating on them they will obviously have enough heals to stay alive for a very long time.