Talk to Maxson to update this quest. You have to go to Fort Strong and wipe out the supermutants in the area. Next, head to the vertibird and man the guns. Take note that the vertibird is not invulnerable and you can get shot down especially by the behemoth below. Concentrate your fire on the behemoth then you can jump off the vertibird and engage the remaining supermutants on the ground.
Once you’ve cleared out all supermutants outside, proceed to the Armory. Clear the building then find the elevator to the SE to reach the lower level of the fort. (Take note that this elevator will be covered in rubble if you visited Fort Strong earlier before taking this mission.) You can find two mini-nukes on the shelf beside the empty mini-nuke shells in an open shipping crate.
Clear the remaining supermutants there then talk to Danse. If he’s not with you, he’ll come to you. Once done, report back to Maxson to complete the mission. You’ll also receive Vertibird Signal Grenades. You can get more of these grenades from Teagan.