I assume you mean the battle at the end??? If you do, then there are 3 part to the battle.
1) You start off on the bottom level, find a place to take cover while you fend off the incoming troops until the doors on the right and left open up.
2) Go up to the 2nd levels where you need to fend off incoming troops again (these are stronger class troops now). Once you killed enough, a troop transport will come crashing through the right wall and clear the path on the right side.
3) Now is the actual fight a Radec, He will be disappearing and reappearing all through the battle, So go back down to the first level where there is more room. Radec will almost always appear behind you or somewhere out of sight (so listen for the sound of him decloaking). Once he appears shoot him until he is out of sight (even when he is cloaked you can still shoot him). After doing this a few times he will appear in front of the main door he entered, just throw a grenade, shoot a rocket, or whatever, and he will finally die.