When QiaoFeng witness DuanYu fight with MuRongFu, he mention that if it's him, he probably would lose to DuanYu too. 六脉神剑 is basically shooting invisible qi from a long range. Enemy can't see the shooting qi, so how would they be able to block. DuanYu pew pew and MuRongRu catches it, his sword exploded. 天龙八部 era has another skill called 生死符, which is a deadly 暗器. Basically you have can only dodge if you try to defend it, most likely you will get hit. "风清扬还提到过独孤九剑的设定:“独孤九剑,有进无退!招招都是进攻,攻敌之不得不守,自己当然不用守了." Which means that you can't even enter 3m radius to DuanYu when he is shooting 六脉神剑, so how would your sword even reach him? So you can only block or dodge. 六脉神剑 requires very high 内功,so don't think will die off easily from exhaustion bah.