If someone is fighting a losing battle, they are trying to do something even when there is little chance of succeeding.

eg: The headmaster is fighting a losing battle trying to ban mobile phones at school.

Note:

Fight a losing battle字面上的意思是打一场必输的战役，含义是做一些几乎没有成功机会的事情，也就是俗话说的死马当活马医。

eg: 班主任试图禁止学生在校使用手机，但根本就是死马当活马医。