It is pretty easy to dose them individually without chasing. Just do it at night when they are well asleep on the roost (wait until kinda late so they are well zonked by the time you get there). Wear a headlamp so you have both hands free, preferably as dim a light as you can still do your work by. With that size flock it may be worth marking them as you do them (various methods come to mind, depending on the color of the chickens) so that if some start jumping down off the roost you don't end up with some double-dosed and others not done at all (you can always do the escapees the next night). But I have had no trouble doing individual things (in my case, dusting for mites) to a group of 10 without chaos.
Good luck,
Pat