To further clarify my thoughts on double team gengar: the amount of time it would take for it to get the necessary boosts, you'd be able to KO it with a strong neutral attack from any xern. Gengar would likely go for sludge bomb to try and take out xern while it can. BTW, which outspeeds? Max speed gengar, or max speed scarf xern? I think xern (scarf or geomancy) would win 1v1, excepting destiny bond (the real danger).
I'd give the timid choice scarf, make the modest a geomancy sweeper, and...not sure what to do with rash, but probably life orb all-out attacker? Not sure what are good checks to dialga, though.