Well in theory L was on the verge of victory.
He had already determined who Kira was (Light), but needed physical proof. He was taking Misa (who knew the identity of Kira) into custody. So between humanity's basic instinct for survival, a lie detector test, and then follow up work on whatever Misa may have said would have had a good chance in resulting in defeat for Light.
However, it took the act of a Death God (which was beyond his human abilities to defeat) to stop him.
I think the minute you add someone, it's L's game...
And realistically Light probably should have lost when that ?Naomi Ex-FBI? agent went to deliver information about who her husband had been doing or whatever information she had (can't remember exactly what she was going to go say). There's no way an Ex-FBI agent would be so dumb as to completely blow their false name and cover to the confident whims of a child. I mean realistically what are the odds someone who has their name kept secret would risk telling anyone (since anyone could be Kira) the sensitive information she had.
But that's just ny thoughts.