How do you make a boss in game maker?

Answer 1

This highly depends on the game, but here is an example:

create:

self.hp = 100;

step:

if irandom(90) = 1

instance_create(self.x,self.y,obj_bullet);

if self.hp <= 0

instance_destroy();

Obviously, you would need to add some kind of attack to reduce its hp.

Answer 2

This is a pretty generic question. Simply have another object, "obj_boss", and have it execute all the boss's events.