Tactical Thinking

Tactical Thinking is a Brotherhood of Steel main quest in Fallout 4.

Detailed walkthrough
Advancing in this quest will make the Sole Survivor an enemy of the Railroad, so if there are any loose ends to tie up with them, do so before speaking to Lancer Captain Kells. Even having this quest active in the log will shut off access to conversation or side quests with some Railroad personnel. Bear in mind, this quest appears automatically at the conclusion of Blind Betrayal. If the player character is not ready to close off activities with the Railroad, avoid turning in that quest for now.

At the start of the quest, the Sole Survivor is informed that Kells wishes to speak to them on the command deck of the Prydwen. In this discussion, Kells orders the Sole Survivor to assault the Railroad HQ and kill the Railroad leaders located there. The list of people to dispose of includes Doctor Carrington, Glory, Deacon, Tinker Tom, and Desdemona.

After agreeing to Kells' orders, head north into the North End neighborhood of Boston, and enter the Old North Church. Resistance from the Railroad is met immediately upon entering the Church. After dispatching all enemies in the church chamber, proceed to the crypt. Several more Railroad personnel attack in the tunnels before the player character can reach the end. When the player character has reached the entrance to Railroad HQ, instead of needing to solve the dial puzzle from Road to Freedom, the Brotherhood of Steel has planted explosives which will blow a hole in the wall and grant the player easy access to the final segment of the mission.

After entering through the destroyed wall and into Railroad HQ, the player character is met with several Railroad threats. While only the people on the list provided in the quest details need to be killed to complete the quest, it may be easier to clear all of Railroad HQ of assailants, save for P.A.M.

After all of the personnel on the list are dead, there are two options for proceeding: the player character may reprogram P.A.M., which is the desire of the Brotherhood, or destroy P.A.M. To reprogram P.A.M., find the P.A.M. mainframe terminal, located near the main area of the Railroad HQ. In the mainframe, use the P.A.M. decryption program holotape that has been provided to successfully reprogram P.A.M. If choosing to destroy P.A.M., skip the steps with the terminal and simply locate her and destroy her. Whichever method is chosen, all of the tasks in Railroad HQ for the quest will be complete.

To complete the quest, one must return and report to Kells on the Prydwen. The Brotherhood will express disappointment if P.A.M. was destroyed rather than reprogrammed. On the other hand, if the player character managed to reprogram P.A.M., she is later transported onto the Prydwen.

Bugs

 * Deacon may appear shortly after the player character enters Old North Church, and if one kills him before the quest tells them to, it may not count towards the objective. Reloading to a save prior to entering Old North Church might fix this. Alternatively, speaking to Lancer Captain Kells once all other objectives are completed may still allow to turn the quest in.
 * Deacon may become unkillable, unselectable in VATS but still shooting back. Reloading to a prior save and killing him with the Mister Sandman perk before obtaining this quest from Lancer Captain Kells might resolve this issue.
 * Deacon may not be encountered inside Old North Church or the Railroad HQ. Instead his quest marker can be found at a random spot anywhere in the Commonwealth. Fast-travelling to a location near his position may only result in him being somewhere else far away upon arrival. Once having caught up with him he can be killed normally. This is more likely to happen if the player character took Deacon's mission Tradecraft prior to taking Tactical Thinking.
 * Detonator may not appear, rendering the mission impossible to finish. By leaving Old North Church, fast travelling to another location, then fast travelling back, Doctor Carrington will appear in the first room of the church. By killing him the detonator should re-appear allowing the player character to continue the quest. He can also be shot through a small patch of wall on the left edge of the unopened secret doorway. VATS will pick him up with a 0% hit chance when you've found it. It is also possible to shoot through the wall with Lorenzo's gamma gun, killing everyone behind it.
 * If the Sole Survivor makes enemies with the Railroad before this mission (for example, performing some missions for The Institute without telling the organization), Deacon may follow the player character somewhere to try to kill him or her (For example: to Red Rocket truck stop). The mission may not be affected though the item on the quest may look unfinished. P.A.M. can be reprogrammed without problems.
 * If one has installed the add-on Far Harbor and discovered Acadia, the player character is forced to tell Kells about Acadia before they can continue this quest.
 * One can skip the "Acadia report" by first reporting Kells about Acadia and response "Nothing else" and immediately pick Kells before reporting again: if it fails, try again until it works.