A spray of bullet hit the tree behind which he was hidding.一

Behind不是多余的,从这句话来看,behind which he was hidding才是作tree的定语.如果按照正常语序来说,这句话可以分解为he was hidding behind the tree,这样你就可以看出,behind是必须地成分.如果你认为不需要behind的话,那这句应该修改为A spray of bullet hit the tree where he was hidding,这里用where作地点状语的话应该省略behind,因为where是副词,要再前面省略介词,而原文中出现which,所以不能省略behind.不知道你有没有清楚呢?

behind which he was hidding才是状语从句，引导词为behind (介词)+which,which先行词为the tree.
这句话分开来写的话就是A spray of bullet hit the tree 和he was hidding behind the tree。behind the tree在从句中做地点状语
整句话译为：一阵扫射出的子弹打中了他...