To make bullets appear, set up some spawner or make an alarm in the boss itself; to make bullets fly at the player, put:
a=instance_create(boss.x,boss.y,bullet1);
a.speed=[speed];
a.direction=point_direction(a.xstart,a.ystart,player.x,player.y);
For making bullets fly in random directions, replace the point_directiok with random(360) or something of the sort.
Depending on what you want the bullet do, you may or may not need to code it separately