I don´t know if this should´ve been fixed with the latest patch, but you can still stagger bosses, till they dissappear.
We were on skittergate and a SF immediately appeared after the first grim. No horde etc.
It climbed up the stairs. At the moment it “hanged around there”, one of us throwed a bomb onto it.
The SF got knocked back into the air (hovered) and dissappered in front of me.