Radec is difficult, but not impossible to kill, especially if you are attempting to get the "Demonslayer" trophy for killing him in twenty minutes. Do not worry about him until you get the cinematic where he says "I guess you want to make it personal!" In the beginning he sends hordes of Helghast troops at you. Just dig in and kill them all. You will be on the ground floor for about three waves of troops. Then the doors on the side will open, allowing you to get to the balcony.

Note: Ammunition is limited on the balcony. Make your shots count. When you go up onto the balcony, there are two flamethrower Helghast waiting. Afterwards there will be two waves of infantry then a mixed group of infantry and knife wielding Helghast officers. Watch out, because when you are on the balcony they will come up from both sides. Take care of them first, then focus on the floor troops. After this wave you are greeted with RPG troops on the far wall of the room. Take them out, then go back to the floor troops. After about six waves you get another cinematic of Radec. You are now back on the ground level. He now phases in and out of invisibility, and shows up beside you and tries to knife you. Keep moving and shooting at him as he shows himself. After hitting him enough times, he retreats to the stairs in front of you. Keep shooting, and when he goes down the battle is over.