Yes, you have to run the perfect Gengar and that's so awesome about Mega Gengar because you tailor him to take out those specific threats! And also, you say Mega Gengar is useless after it takes out one pokemon. I disagree. If you have some life left after you use Perish Song, you might be able to use it again. For 3 atks + Destiny Bond, you can start spamming Shadow Ball since Ghost has good coverage now. You'll be doing damage to another 'mon at least. And the perish song set is mainly for stall teams and walls. Walls aren't normally fast since they're invested in HP and Def/SpD respectively, so Mega Gengar is commonly faster. Thus, you can trap, use Sub, then Protect, then switch out for the kill. And as for Gyro Ball, you CAN spam it, but if I use Sub and Protect, you're not hitting me right? Also, if your Skarmory has Sturdy and I get it down to 1% I don't think you'll be guaranteed that free switch in later on considering on the prevalence of Stealth Rocks, and how Mega Gengar is used to aid sweepers, aka I might be using attacking moves when your Skarmory switches in and it dies before getting the Wish. And yeah, a lot of teams can handle Mega Gengar depending on the set. The 3 Atks + Destiny Bond does well against offense, and PS + ST against stall, but I would say that due to the prevalence of offense, I would run 3 Atks + Destiny Bond. Or if you're desperate for something against stall walls, you can just use the PS + ST set. But in the end, that doesn't matter, since Mega Gengar is just coverage for your team to handle a specific 'mon. Also, Mega Gengar doesn't necessarily need to take damage when switching in to a 'mon. I commonly use CB 252 Hp / 252 Atk Scizor to tank a hit, then use U-turn before switching in Mega Gengar to switch, or other slow yet powerful pokemon to have my Mega Gengar get a free switch in.