Every now and then (between 6 - 14 seconds) a MINIBOSS will appear, after killing 4 of them, a BOSS will appear. kill him and then you'll need to kill another 4 MINIBOSS's. it just keeps repeating like that.
Like i said, basic.
There are 3 types of powerups that will periodically appear:
Nuke - Destroys all NORMAL enemies on screen.
Shield - Destroys all NORMAL enemies and al Missiles that contact you. damages BOSS's and MINIBOSS's
Health - Full hp.
That should tell you all you need to know, the rest is pretty straightforward.
UPDATES COMING SOON