you only need to kill about 10 borg. put your team at the entrance and shoot the first few round the corner. once they are dead run down, shot a couple more by the assimilated undine, then run back to your team. let the borg come and kill them. that should be enough to spawn the undine.
let the borg come to you. dont go running after them or you will start to agro the entire room.
once the dialogue about the undine pops up in the task bar, get away from the fighting and let the next pop up appear and he will then comae after you.