To literally shed skin and lose flesh; to suffer extreme physical exhaustion or trauma.

这几个月的艰苦生活，让他整个人都脱皮掉肉，瘦了一大圈。

To work as hard as possible; to exert oneself to the absolute limit; to work one's butt off.

只要有这种脱皮掉肉的干劲，就没有什么难关是攻不破的。