I see a lot of people questioning why you would use Perish Song on Mega Gengar when it has fantastic offenses but low bulk. Well, I understand why you would not be convinced about the Perish Song + Shadow Tag combination on a Pokemon like Gengar, but you have to realize how strong it can be. As long as the Pokemon that is out against you can't do 75% or more in one turn, is slower than you, and has no priority, you can just Perish Song - Protect - Substitute, and then kill them. While achieving all of those expectations may seem tough, remember Mega Gengar as a Pokemon with Shadow Tag by nature has more control over these situations than the opponent.
The Perish Song trapping set also does not get affected by the single turn you do not have Shadow Tag + 130 base Speed as much as an offensive set because it has Protect, although you could do an offensive set with Protect if you really wanted to (actually not that bad on Gengar -- it already used Protect rarely in the past generations for certain Pokemon like Scizor).
Anyways, the Perish Song set does better vs defensive Pokemon as believe it or not, Mega Gengar might not have enough firepower to break through certain walls.
Personally my favorite set is Shadow Ball / Sludge Bomb / Focus Blast / Destiny Bond. Destiny Bond is actually a really nifty move with that sexy Speed stat as it allows you to kill off Pokemon you cannot OHKO (which happens fairly often with Mega Gengar despite that 170 base Special Attack). It seems like a situational move, but it actually comes into play pretty damn often.