Well, my own strategy against Narcissa was not to use Dark-types, but to paralyze all of her Pokemon and use Hex on them later. There can be other ways to beat her, of course, but I'll likely think the most about this one when giving suggestions... So yeah, I'll be just praying the move Hex from now on :]
Of the Pokemon you have on your team, the best choices to bring against Narcissa would most likely be Chesnaught, Garbodor and Dusclops:
Chesnaught with Bulletproof is essentially immune to Shadow Ball and Sludge Bomb (so it makes Gengar almost useless); the problem is that it can't do much damage in return - as such, you can try to stall Gengar and wear it down slowly. Leech seed could have helped...
Garbodor can poison some of Narcissa's Pokemon either with Toxic or Toxic Spikes - so a good strategy could be to lead with it and use toxic spikes, then switch out in order to use Toxic on all of the Pokemon that have Levitate as they are sent out.
Dusclops should learn Hex at level 40: that is a fantastic move to use in this gym as it's boosted by the field, is super effective against all Pokemon bar Spiritomb, and has a great 130 Power (not counting Stab) on statused enemy Pokemon. But the best part is, that Garbodor is here just for that!
About the rest of the team... Swoobat is at a type disadvantage; Ambipom has nothing to hit Ghost-types while they do, thanks to the field; Torkoal and Seismitoad can still be used to dish out some damage, but won't probably last that long; and Forretress still can't do much offensively, like Chesnaught.
Here there are also some other Pokemon suggestions:
If Dusclops doesn't hit hard enough, you can still use some other Ghost-type Pokemon like Yamask. It's found right inside Narcissa's Gym. Cofagrigus has a way better Special Attack than Dusclops (base 95 rather than base 60), learns Will-o-wisp and Ominous Wind (which has a higher chance of raising the user's stats, if I remember correctly), and evolves at level 34 from Yamask. If they are available, try to get one at level 32 so that it still knows Hex. Its ability, Mummy, may become useful against certain physical attackers (Medicham for example), and is about as bulky as Dusclops, at least from the Special side.
Both Cofagrigus and Dusclops are still very slow and will inevitably take a lot of damage: to outspeed Narcissa's Pokemon, the best Pokemon to use is probably Misdreavus. It's obtainable in Gearen City at the start of the game - this means that you can only get it at level 7 and then train it until it gets to about level 40 - and it's another Pokemon that can use Hex. While not as resilient as the other ghost-types, it's a lot faster and has some reasonable Special Attack. It can also evolve into Mismagius right before Narcissa, however, it's better to wait until it learns moves like Shadow Ball (level 41) and, if you want, Perish Song (level 46).
If the problem is that Narcissa's Pokemon are too fast, you could paralyze them (with Thunder Wave) rather than poisoning them, with electric types such as Zebstrika and Manectric (Electrike is now available in Chrysolia Hotel; Blitzle is in Amethyst Grotto). These Pokemon can also be useful to take care of Drifblim - although it may carry a Berry to weaken Electric-type attacks; I don't know if that happens only on Intense difficulty. After that, you can hopefully switch to your Ghost-type and use Hex, which will do a lot of damage (my Impish [hinders Special Attack] Misdreavus knocked out in one hit Gengar, Chandelure, and Mismagius)
Dark-type Pokemon resist Ghost-type attacks, but thanks to the field, can't do much damage in return, and as such I'd suggest to use them only on Defensive teams.
Well, I hope I was helpful