For some interesting calculations, check out the following. I am an avid supporter of Ghost / Fighting, and I would like to show just how awesome it would be. However, to properly display damage calculations, I will have to assign arbitrary numbers to the base statistics that I feel would be good choices.
90 HP / 82 ATK / 125 DEF / 78 SPATK / 100 SPDEF / 65 SPD
Base Statistics Total = 540
Let us presume we used a very basic 252 HP / 252 Defense +Defense Spread, because we want this to be able to simply overcome certain physical attackers.
Behold some of these damage calculations against the two main Pokemon that it could counter, making it one of the only Pokemon in the game that can reliably counter both Heracross and Tyranitar. Hell, according to the calculations, a Ghost / Fighting would perhaps convince most Heracross to carry Shadow Claw over Night Slash or Pursuit!
Against 252 HP / 252 Defense +Defense Ghost / Fighting
Max Attack Choice Band Heracross Close Combat = Immune
Max Attack Choice Band Heracross Megahorn = 12-15%
Max Attack Choice Band Heracross Stone Edge = 14-16%
Max Attack Choice Band Heracross Pursuit (Without Switching) = 11-14%
Max Attack Choice Band Heracross Night Slash = 20-23%
Max Attack Choice Band Heracross Shadow Claw = 40-47%
Max Attack Choice Band Tyranitar Stone Edge = 22-26%
Max Attack Choice Band Tyranitar Crunch = 35-42%
Max Attack Choice Band Tyranitar Earthquake = 30-35%
And, just to prove that this would not be overpowered, here are some damage calculations that show that this could be taken down due to its Flying, Ghost, and Psychic weaknesses. Keep in mind that I am only addressing physical attacks, as it would obviously have a far greater weakness to special attacks. So, remember, all of the following calculations are what certain Pokemon could do against an absolute Max Physical Defensive Ghost / Fighting with the given arbitrary statistics.
Max Attack Choice Band Medicham Psycho Cut = 74-87%
Max Attack Choice Scarf Medicham Psycho Cut = 50-59%
Max Attack Choice Band Staraptor Brave Bird = 98-100%
Max Attack Choice Scarf Staraptor Brave Bird = 66-77%
Just giving out some information. Basically, a Ghost / Fighting would have some problems countering some of the premier special attackers in the game due to the prevalence of Gengar (Shadow Ball), Yanmega (Air Slash), Togekiss (Air Slash), Lucario (Shadow Ball), Alakazam (Psychic / Shadow Ball), and the like, as well as some physical attackers like Medicham (Psycho Cut) and Staraptor (Brave Bird).
Yet another chapter in the case for Ghost / Fighting ...