Pretty sure the most reliable trapping set should be something like Shadow Ball / Sludge Bomb / Taunt / Destiny Bond, with slight differences based on what you need to trap (Focus Blast for Ferrothorn/Heatran, Thunderbolt for bulky Water-types, etc). Destiny Bond combined with Taunt is a really great way to make sure you can take out anything (probably some wall or tank) keeping you from sweeping with a teammate. The best part about this set is that now that Ghost hits neutrally against Steel-types, Ghost/Poison has phenomenal neutral coverage, hitting everything barring Bisharp, Drapion, Krookodile, Skuntank, and Tyranitar for neutral damage or better. Hell, even without its mega stone, previous sets such as SubDisable and SubSplit just got that much more annoying because Sludge Bomb hits just a bit harder than Focus Blast against neutral targets (and comes with a really solid PSN chance), and Shadow Ball cleans out previous checks such as Jirachi and new things that would have been able to check it in BW such as Aegislash. Gengar pretty much gets better and better every generation, but in this generation, it just got nasty.