Half demon Half demon sider?... Walk into the battle tossing grenades. She will use her powers to throw things at you and toss you across the room. Try to aviod the flying objects and fill her with whatever you got. You will encounter her later on in the game this time with tons of Trites and second of herself. Long-Range is about the only way to fight this battle. Cause you can't get up close enough to use the shotgun.
You encounter the Gaurdian at the end of hell. He has no vision so he uses seekers to do his seeing. Destroy the seekers with everything you got. You do not want him to give you a trashing up-close. Once the seekers are destroyed empty your most powerful weapons into the crystal like think on his back. The seekers will respawn and it will continue til the beast falls. The reqard for winning is the soul cube.
This mutant demon/tank thing. Is what the sarge used to be. Now a demonic tank with his own BFG. He is surprisingly easy. He will charge you simple avoid and fire rockets or grenades at him. Also look out for BFG shots those do quite a bit of damage. Once he falls you will now have the BFG.
The last and final boss of the game is one of the easiest foes to face. I recommend simply running around in cricles around the gate to hell killing everything that crawls out of it. The Cyberdemon will fire rockets at you but as long as your staying ahead of him you should be fine. Killing all the monsters will charge your soul cube and you can use it to take out the cyberdemon with ease.