I tend to lead with either Talonflame and Ferrothorn or Swampert and Ferrothorn. I use Ferrothorn to spam Thunder Wave on either special attackers of bulky Pokémon, and use Swampert or Talonflame to try and burn any physical attackers I see. The two together can either get some chip damage or even KOs before fainting.
After one of the two leads faint, I bring in Clefable, and usually stall with Protect and get some chip damage with Moonblast until my other lead dies, as I like to keep it as healthy as possible until Salamence comes out. When Clefable and Salamence are out, I'll either spam with Helping Hand + Hyper Voice, or go for Moonblast + Draco/D-E on a single Pokémon if it's being a nuisance
As for Gengar, I've only had about 7 battles, so I haven't really had the chance to try it out yet. If I'm honest, I threw him in because I couldn't think of a 6th member and he was in the recommendations as a partner for one of the other Pokémon, but I was too lazy to read the whole paragraph explaining how to use them together.
I tend to also use Swamper over Talonflame more often, so I never use Tailwind/Taunt as I feel Will-O-Wisp is more important to go for first. I also don't see Taunt being needed that much as the VGC meta-game is more about Protect/attacking rather than setting up (it is essential for Thundurus/other status spreaders, though).
Overall, I kind of threw this team together without doing too much research and the above strategy is what I came up with on the spot. My streak so far is 5 wins and 2 losses, so I suppose it's not too bad. I haven't been put in a lot of different situations yet, so I haven't seen that many checks to my strategy. My only real problem is Steel types that wall Salamence and Clefable after Swampert has left the field.