Precipice of War

The BoS will attack from the escape tunnel. Kill them all then talk to Desdemona. Next, head through the catacombs and find Glory in her dying breath. She'll succumb to her wounds so proceed through the catacombs and clear out more enemies until you reach the church itself.

If you have the Penetrator Perk, you can snipe the fusion cores of the Power-armored BoS soldiers and force them to eject. This will allow you to equip their Power Armors and use it against them. The BoS soldiers are also equipped with heavy laser or ballistic weapons that you can scavenge in case you don't have enough protection or firepower. Clear the church then talk to Desdemona to update your quest objectives.

Next, you have to travel to the Cambridge Police Station. Don't fast-travel to it directly; find a map marker nearby to fast-travel in, like College Square for example, then walk to it to avoid getting teleported in the middle of the Brotherhood forces stationed in the police station. Meet up with Tom and Deacon and clear out the hostiles around the police station.

Make your way inside the station itself and clear more enemies. If you didn't kill Clarke in the quest Duty or Dishonor, you can find him locked up in one of the cells in the station. He's not hostile so you can free him if you want. You have to kill all enemies, including the one in the police station's motorpool outside the station. Clear the last remaining enemies then talk to Tinker Tom to learn the plan and get the explosives. Head to the rooftop and defend Tinker Tom as he preps the vertibird for flight. Once clear, talk to Deacon to complete the quest and start the next one.