I think there should be a D/P McGar set, because Blissey and to a lesser extent Snorlax walls most of Gengar's attacks unless Gengar has Choice Specs and unless Gengar's Focus Blast actually hits.
Addition to Analysis:
http://www.smogon.com/dp/pokemon/gengar
[SET]
name: DP McGar
move 1: Substitute / Explosion
move 2: Focus Punch
move 3: Shadow Ball
move 4: Thunderbolt
item: Leftovers / Life Orb
nature: Rash
evs: 156 Atk / 162 SpA / 192 Spd
[SET COMMENTS]
<p>This moveset serves the same purpose as McGar did in Advance: being a wall-breaker, hitting the special walls - Blissey, Snorlax and Regice - while hitting Skarmory and bulky waters hard with Thunderbolt. And now Gengar has an additional advantage: a special-based STAB Shadow Ball. The pesky Swampert and Metagross that previously stopped Gengar in its tracks now takes up to 56% and 62% from a Life Orb Shadow Ball respectively. Focus Punch guarantees a 2HKO with Life Orb on Blissey and a 2HKO with Leftovers if Stealth Rock is up, and 2HKOs Snorlax with Life Orb. Thunderbolt hits everything resistant to Shadow Ball relatively hard, finishing off low health targets quickly without having to set up with Substitute / Focus Punch. </p>
<p>The problem with using Life Orb, however, is that Substitute + Stealth Rock + Life Orb + Sandstorm quickly adds up. On the other hand, without Life Orb Gengar eschews the ability to 2HKO Garchomp, Snorlax, Suicune among many others. This leaves two alternatives: either use Substitute with Leftovers, sacrificing power for survival; or use Life Orb with Explosion by predicting enemy Blissey and Snorlax when using Focus Punch, wreaking havoc in general, and when Life Orb recoil finally gets to you, by Exploding.</p>
[Other Options]
<p> Hidden Power Ice could go over Thunderbolt to solve a Garchomp / Salamence problem. </p>
[EVs]
<p>DP McGar has 192 Spd to reach 304 Spd outspeeding Max Speed Adamant Garchomp, at absolute minimum. Generally 304 speed is enough as you'll want your attacks to have maximized power but you could cut a few EVs off the Special Attack stat, use a +Spd nature and run 216 EVs putting you ahead of Mismagius and Jolly Garchomp at 341 speed.</p>
[Counters]
<p>There's not much that can take less than 40% damage when switching into this Gengar. Ice Beam Blissey can beat it one on one by breaking Gengar's Substitutes. Sp. Def orientented Bronzong, Jirachi and Cresselia can take Gengar's attacks and Psychic back, and to a lesser extent Spiritomb can stop and hurt Gengar with Sucker Punch and Pursuit.
Addition to Analysis:
http://www.smogon.com/dp/pokemon/gengar
[SET]
name: DP McGar
move 1: Substitute / Explosion
move 2: Focus Punch
move 3: Shadow Ball
move 4: Thunderbolt
item: Leftovers / Life Orb
nature: Rash
evs: 156 Atk / 162 SpA / 192 Spd
[SET COMMENTS]
<p>This moveset serves the same purpose as McGar did in Advance: being a wall-breaker, hitting the special walls - Blissey, Snorlax and Regice - while hitting Skarmory and bulky waters hard with Thunderbolt. And now Gengar has an additional advantage: a special-based STAB Shadow Ball. The pesky Swampert and Metagross that previously stopped Gengar in its tracks now takes up to 56% and 62% from a Life Orb Shadow Ball respectively. Focus Punch guarantees a 2HKO with Life Orb on Blissey and a 2HKO with Leftovers if Stealth Rock is up, and 2HKOs Snorlax with Life Orb. Thunderbolt hits everything resistant to Shadow Ball relatively hard, finishing off low health targets quickly without having to set up with Substitute / Focus Punch. </p>
<p>The problem with using Life Orb, however, is that Substitute + Stealth Rock + Life Orb + Sandstorm quickly adds up. On the other hand, without Life Orb Gengar eschews the ability to 2HKO Garchomp, Snorlax, Suicune among many others. This leaves two alternatives: either use Substitute with Leftovers, sacrificing power for survival; or use Life Orb with Explosion by predicting enemy Blissey and Snorlax when using Focus Punch, wreaking havoc in general, and when Life Orb recoil finally gets to you, by Exploding.</p>
[Other Options]
<p> Hidden Power Ice could go over Thunderbolt to solve a Garchomp / Salamence problem. </p>
[EVs]
<p>DP McGar has 192 Spd to reach 304 Spd outspeeding Max Speed Adamant Garchomp, at absolute minimum. Generally 304 speed is enough as you'll want your attacks to have maximized power but you could cut a few EVs off the Special Attack stat, use a +Spd nature and run 216 EVs putting you ahead of Mismagius and Jolly Garchomp at 341 speed.</p>
[Counters]
<p>There's not much that can take less than 40% damage when switching into this Gengar. Ice Beam Blissey can beat it one on one by breaking Gengar's Substitutes. Sp. Def orientented Bronzong, Jirachi and Cresselia can take Gengar's attacks and Psychic back, and to a lesser extent Spiritomb can stop and hurt Gengar with Sucker Punch and Pursuit.