This was an idea for a flashback as I pick up my story around 100 trillions years from now at the point where star formation is ceasing and the red dwarf and stellar remnant era begins. Now living in an era where abundant stellar energy is decreasing my characters in an almost comedy moment look back at memorable times in the past including the time nearly the whole galaxy co-ordinated their stars to fire beams together in one overpowered attack at a galactic threat.
I want to have an over showy moment where nearly a whole super galaxies worth of stars are turned to Nicoll-Dyson beams to fire at an immensely massive treat.
The advanced civilization will set time in the future to launch all beams, as the speed of light limitations wouldn't allow communication without some locations needing to wait thousands of years for an update and for this question we can ignore why a target would be in the right place in around a billion years for all beams to strike around the same time period (I am currently thinking of an intergalactic war an the enemy has sent a giant, close to invincible void creature)
Further out stars would have to shoot their beam a few thousand years before stars that are closer to the target but I am unsure of the other elements needed for the formation.
Is it possible for a galaxies worth of beams to strike a single target or will momentum, interstellar/galactic medium, or other physics or technical reasons make this a difficult or impossible task?