Do not worry about him until you reach the cinematic where he says "I guess you want to make it personal!" At the start, he sends hordes of Helghast troops at you. 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: Ammo is limited on the balcony. Make your shots count. When you go up onto the balcony, there will be two flamethrower Helghast waiting. Afterward, there will be two waves of infantry, followed by a mixed group of infantry and knife wielding Helghast officers. Watch out; 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 next to you and tries to knife you. Continue 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.
After he makes the comment about "making it personal", waves of troops will then continuously appear. There is no real strategy here other than "run and gun". When it gets down to just you and Radec and he keeps turning invisible, stock up on ammo (rifle recommended). Go up to the balcony and put your back in a corner. Radec will not be able to sneak up on you. You only have to cover left and right. You will hear him appear and see a flash of light, indicating which direction he is coming from. Sometimes he will appear on the ground floor. Peek over the rail and shoot him a few times, then get back to the corner. After a short while he will appear on the stairs. Finish him off and you will have defeated him.