When confronting the final boss,(Nemesis' final transformation) make him retreat into the corner from where he came from by your rocket launcher, and grenades, then you have to get the cannon online by PUSHING IN the three power sources, marked 1,2,3. When that is done, fire the cannon to dispose of the debris, then it will auotmatically fire again, at Nemesis. You then have the choice to finish him off yourself(this is an FMV!) or just running.