Alright, I tried using the draw event, which I've never used before, and ran into some problems
I only wrote this:
draw_sprite(HPBar,0,10,10)
and neither the sprite nor the boss appears.
also, just now, i tried
hpbar=draw_sprite(HPBar,0,10,10);
hpbar.image_xscale=500/20*hp
the boss and hp bar did not show, and the players sprite got stretched instead.
EDIT: Oops, didn't see your edit. Tried it, and it still didn't work. what should I put for the image scaling if the max hp is 20 and I want the length to be 500? I think the fact that I put "500/20*hp" is what screwed everything up.
EDIT2: Wow, I'm incredibly stupid. I replaced spr_healthbar with my sprite name, and that fixed it. But the boss still doesn't seem to want to exist. By that I mean it's invisible. It's still shootable if you can guess where he is.
EDIT3: Once again, I'm terrible. I just put in draw_self(); before the other line and everything worked perfectly.