EVs would totally be helpful here.
Anyway, Gengar needs a Timid nature to outspeed as much as it can. It can also get away with just Shadow Ball and Focus Blast as its lone offensive moves; Ghost + Fighting is unresisted:
-Substitute
-Shadow Ball
-Focus Blast
-Disable/Pain Split/Thunderbolt/Hidden Power (Fire)
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Leftovers/Black Sludge/Life Orb
For Jellicent, Substitute and Water Spout clash; the latter move gets stronger the more HP Jellicent has, and using Sub drains the user's HP, weakening Water Spout. Jellicent is also bulky enough, so it doesn't need Sub much at all. Water Spout is also better off in offensive sets:
-Scald/Surf
-Will-o-Wisp/Toxic
-Recover
-Taunt
Nature: Bold
EVs: 248 HP/216 Def/44 Spe
Item: Leftovers
or
-Scald
-Will-o-Wisp/Toxic
-Shadow Ball/Ice Beam
-Recover
Nature: Calm
EVs: 252 HP/36 Def/220 SDef
Item: Leftovers
Ability: Water Absorb
or
-Water Spout
-Shadow Ball
-Surf/Scald/Hydro Pump
-Ice Beam/Giga Drain
Nature: Modest
EVs: 172 HP/252 SAtk/84 Spe
Item: Choice Specs
Ability: Water Absorb
Banette isn't good honestly, except be a suicide Trick Room user. Using a Jolly nature ruins the point of Trick Room, since Jolly boosts Speed, and Trick Room makes slow Pokemon go first temporarily. Ideally, a Banette set should look like this:
-Trick Room
-Shadow Claw
-Destiny Bond
-Taunt/Will-o-Wisp
Nature: Brave
EVs: 252 Atk
Item: Focus Sash
Ability: Insomnia/Cursed Body
While Dusclops is better than Dusknoir in terms of defensive utility, Dusknoir has better offensive presence. But IMO, I think you should go with Cofagrigus because it has high defenses while still having decent offenses:
-Trick Room
-Nasty Plot
-Shadow Ball
-Hidden Power (Fighting)
Nature: Quiet
EVs: 252 HP/4 Def/252 SDef
Item: Leftovers
or
-Shadow Ball
-Will-o-Wisp
-Haze
-Pain Split
Nature: Bold
EVs: 252 HP/136 Def/120 SDef
Item: Leftovers
or
-Calm Mind
-Shadow Ball
-Hidden Power (Fighting)
-Rest/Pain Split
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Chesto Berry/Leftovers
Spiritomb is a decent choice because of its lack of weaknesses and high defenses. However, it suffers from a lack of resistances, other than Poison, Psychic, Normal and Fighting (Spiritomb is immune to the latter 3 types):
-Sucker Punch
-Shadow Sneak
-Pursuit
-Trick/Will-o-Wisp
Nature: Adamant
EVs: 172 HP/252 Atk/84 Spe
Item: Choice Band/Blackglasses
Ability: Pressure
or
-Calm Mind
-Dark Pulse
-Rest
-Sleep Talk
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Pressure
or
-Will-o-Wisp
-Pain Split/Rest
-Sucker Punch
-Shadow Sneak
Nature: Impish
EVs: 252 HP/192 Def/64 SDef
Item: Leftovers
Ability: Pressure
Other Ghost Pokemon to consider include Chandelure (for its massive Special Attack and decent offensive movepool), Sableye (for Prankster and good support movepool), Froslass (for speedy Spikes stacking) and Golurk (for its high Attack and good move pool):
Chandelure:
-Substitute
-Fire Blast
-Shadow Ball
-Hidden Power (Fighting)/Energy Ball/Pain Split
Nature: Modest/Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Leftovers/Life Orb
Ability: Flash Fire
or
-Fire Blast/Overheat
-Shadow Ball
-Energy Ball
-Hidden Power (Fighting)/Trick
Nature: Modest/Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Choice Specs/Choice Scarf
Ability: Flash Fire
Sableye:
-Taunt
-Will-o-Wisp
-Recover
-Foul Play/Night Shade
Nature: Careful/Calm
EVs: 252 HP/120 Def/136 SDef
Item: Leftovers
Ability: Prankster
or
-Trick
-Recover
-Taunt
-Will-o-Wisp/Night Shade/Foul Play/Toxic
Nature: Calm/Careful
EVs: 252 HP/120 Def/136 SDef
Item: Lagging Tail/Iron Ball
Ability: Prankster
or
-Calm Mind
-Dark Pulse/Shadow Ball
-Hidden Power (Fighting)/Will-o-Wisp
-Recover
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Prankster
Froslass:
-Taunt
-Spikes
-Destiny Bond/Pain Split
-Ice Beam/Shadow Ball
Nature: Timid
EVs: 248 HP/8 SDef/252 Spe
Item: Leftovers
Ability: Cursed Body
or
-Spikes
-Taunt
-Destiny Bond
-Ice Beam/Shadow Ball
Nature: Timid
EVs: 252 HP/4 SDef/252 Spe
Item: Focus Sash
Ability: Cursed Body
Golurk (B2/W2):
-Substitute
-Focus Punch
-Earthquake
-Shadow Punch
Nature: Adamant
EVs: 172 HP/252 Atk/84 Spe
Item: Leftovers
Ability: Iron Fist
or
-Earthquake
-Shadow Punch
-Drain Punch
-Ice Punch
Nature: Adamant
EVs: 172 HP/252 Atk/84 Spe
Item: Choice Band
Ability: Iron Fist
or
-Stealth Rock
-Earthquake
-Shadow Punch
-Ice Punch/DynamicPunch
Nature: Adamant
EVs: 176 HP/168 Atk/164 Spe
Item: Leftovers
Ability: Iron Fist/No Guard
or
-Rock Polish
-Earthquake
-Shadow Punch
-Ice Punch/Stone Edge/Drain Punch
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb
Ability: Iron Fist