unassailable
adj
(a) that cannot be attacked or conquered 攻不破的; 不可征服的:an unassailable stronghold, fortress, etc 攻不破的堡垒、 要塞等
Liverpool have (built up) an unassailable lead at the top of the First Division. 利物浦队高居甲级队之首, 所向无敌.
(b) (fig 比喻) that cannot be questioned or refuted 不容置疑的; 无可辩驳的
Her position/argument is unassailable. 她的立场[论点]是无懈可击的.