With Our Powers Combined

With Our Powers Combined is a quest in Fallout 4. It occurs after the ending if the player finishes The Nuclear Option & Old Guns and has (or gains) hostile status with the Brotherhood of Steel.

Although this is a Minutemen quest, it can be received even if the player finished the main quest by siding with the Brotherhood of Steel.

Quick walkthrough
Build artillery defenses after speaking with Preston. Build an artillery piece and later assign a settler to man it in five different settlements the Sole Survivor owns.

Once that is done, visit the Minutemen radio operator in the Castle and ask him to give the strike order. Prepare for the attack and defeat the enemy.

Bugs
Open up console using ~ (or another key on a non-US keyboard) Select the radio transmitter (hex value should be 0009FF48[EP]) Type (without quotes): "setpapyrusvar bAllowPlayerAssignment True" Assign any settler to the radio transmitter; this should force the minute man to sit down and start using it again. This fix cannot be applied to consoles so saving before quest is advisable. .
 * There is a possible bug that makes it impossible to progress further in the story due to the radio man not launching artillery, and saying that the radio transmitter needs more electricity. To fix: