To kill Radec to have to switch between guns when he is shooting at you at a distance, and your knife when he comes for you with his knife. Do this trying to anticipate him giving the first slash and then continue until he goes away to shoot you. You also better keep your back safe against a wall or better a corner because he will try to stab your back. Good luck!
Definitely back into a corner - the narrow ground level corridor worked OK for me - that he may only come at you from the front, and stab-stab-stab at him with your knife. Once he backs off to shoot at you, get out your own gun. What began as a seemingly hopeless business ended up being suddenly far from difficult ....on a not too high level of difficulty I must add.