Destroy All of One One object on collision

Let the thing that destroy the blocks ( I'll call it destroyer ) have a script that stores an array of transform variables. As soon as any block is instantiated, the first thing you do is add its transform to the array being stored in script of the destroyer.

Next, the destroyer detects when any block hits it, this can be done using OnCOllisionEnter function. As soon as a block hits the destroyer you access all the elements stored in the array using a for loop and destroy them one by one by running the loop required number of times