Like HOD, he first thing you should know about HOD2 bosses, is their weakness.
Since you don’t know The Emperor’s weak point, shoot at the red circle on his chest
At first, he’ll use he’s balls to attack you (lol), in order to destroy the balls and save HP shoot at them, then The Emperor will attack you with his sword, when his HP will be about this:
He will use his balls (lol again) to summon old bosses that you defeated, in order to kill them shoot at their weakness point:
After you kill some of the bosses, The Emperor will turn into a little red flying ball, that is his weakness point, shoot it and kill The Emperor.