How can I kill this beast?

I am in the second Hell level and I think this is the Cyberdemon in front of me. How can I kill it? The whispers tell me to shoot the guardians, but whenever I kill all of the floating things, new ones are created by the Demon. Also, the crosshair doesn´t turn red when I point at the Demon. What to do?

That's no cyberdemon, that's the guardian. Kill all the floaties then shoot the blue ball that appears ontop of the guardian. Rinse and repeat.

Thanks, I´m gonna try this.