Gengar is better off with a Timid nature so it can outspeed as much as it can. Hypnosis + Dream Eater is tempting, but the opponent can just switch out to render Dream Eater useless. It's also currently walled hard by Steels and it desperately needs Focus Blast to deal with those types; also Ghost + Fighting is unresisted:
-Substitute
-Shadow Ball
-Focus Blast
-Disable/Hidden Power (Fire)/Thunderbolt/Pain Split
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Black Sludge/Life Orb
or
-Shadow Ball
-Focus Blast
-Hidden Power (Fire)
-Protect
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb/Black Sludge
Alakazam must always use Magic Guard as its ability, since it prevents passive damage; otherwise, don't bother:
-Psychic/Psyshock
-Shadow Ball
-Focus Blast
-Hidden Power (Ice/Fire)/Substitute
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Focus Sash/Life Orb
Ability: Magic Guard
Otherwise, use Reuniclus if you can't get Magic Guard Abra from the Dream World:
-Trick Room
-Psychic/Psyshock
-Shadow Ball
-Focus Blast/Hidden Power (Fire)
Nature: Quiet
EVs: 192 HP/64 Def/252 SAtk
Item: Life Orb/Leftovers
Ability: Magic Guard
or
-Calm Mind
-Psychic/Psyshock
-Focus Blast
-Recover
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Life Orb/Leftovers
Ability: Magic Guard
Venusaur must always be paired with Ninetales; otherwise, it's mediocre setting up its own sun, given that weather doesn't last very long (even though Ninetales only uses Sunny Day to win weather wars, and this isn't a weather team). Use Ferrothorn for its high defenses:
-Stealth Rock/Spikes
-Leech Seed
-Power Whip
-Gyro Ball/Thunder Wave/Protect
Nature: Relaxed
EVs: 252 HP/88 Def/168 SDef
Item: Leftovers/Shed Shell
While Garchomp doesn't have a good movepool compared to its fellow pseudo-legends, it still has enough coverage moves to get by. Having the first 3 slots filled in by offensive moves of the same type is bad, unless it's for utility purposes (just look at Skuntank):
-Outrage
-Earthquake
-Fire Fang/Aqua Tail
-Dragon Claw/Dual Chop
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band
or
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Focus Sash
or
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
-Fire Fang/Aqua Tail
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Yache Berry/Haban Berry/Lum Berry
or
-Substitute
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Leftovers/Salac Berry
or
-Outrage
-Earthquake
-Stone Edge/Aqua Tail/Fire Blast
-Dragon Claw/Dual Chop
Nature: Jolly/Naive
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Scarf
or
-Stealth Rock
-Dragon Tail/Dragon Claw
-Earthquake
-Fire Blast/Aqua Tail/Protect
Nature: Jolly
EVs: 252 HP/196 Def/60 Spe
Item: Leftovers
Tyranitar is normally better off on dedicated sand teams, but given that SubSplit Gengar, Ferrothorn, Magic Guard Alakazam/Reuniclus (and Garchomp if Sand Veil clause isn't in effect) benefit from Sandstorm, it might be worth a try:
-Stealth Rock
-Crunch
-Pursuit
-Fire Blast/Superpower/Stone Edge
Nature: Sassy/Careful
EVs: 252 HP/4 Atk/252 SDef
Item: Leftovers
Ability: Sand Stream
or
-Stone Edge
-Crunch
-Pursuit
-Superpower
Nature: Adamant/Jolly
EVs: 180 HP/252 Atk/76 Spe (Adamant) or 4 HP/252 Atk/252 Spe (Jolly)
Item: Choice Band (Adamant)/Choice Scarf (Jolly)
Ability: Sand Stream
Since Zoroark and Tyranitar share weaknesses, you'll have to decide between one or the other. Normally, Zoroark is better as Timid, but Hasty's close enough
Zoroark:
-Sucker Punch
-Low Kick
-Flamethrower
-Dark Pulse/Hidden Power (Ice)
Nature: Naughty
EVs: 200 Atk/92 SAtk/216 Spe
Item: Life Orb
or
-Dark Pulse
-Focus Blast
-Flamethrower/Hidden Power (Ice)
-Trick
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Specs
or
-Nasty Plot
-Dark Pulse
-Focus Blast
-Flamethrower/Substitute
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Focus Sash/Life Orb
or
-Swords Dance
-Sucker Punch
-Low Kick
-Flamethrower/Night Slash
Nature: Naughty
EVs: 252 Atk/40 SAtk/216 Spe
Item: Life Orb
While Swampert still sees use in OU, it's still too slow to utilize a Jolly nature; also, don't use Hyper Beam-style moves ever competitively, since they leave the user a sitting duck the next turn:
-Waterfall
-Earthquake
-Superpower
-Stone Edge/Ice Punch
Nature: Adamant
EVs: 172 HP/252 Atk/84 Spe
Item: Choice Band
Otherwise, you're better off with Gastrodon, mainly because of its access to recovery and ability to check opposing Water Pokémon:
-Earthquake/Earth Power
-Scald/Ice Beam
-Toxic
-Recover
Nature: Sassy/Calm
EVs: 252 HP/84 Def/172 SDef
Item: Leftovers
Ability: Storm Drain
or
-Surf
-Earth Power
-Ice Beam
-Hidden Power (Grass)/Recover
Nature: Modest
EVs: 124 HP/252 SAtk/132 SDef
Item: Choice Specs/Life Orb
Ability: Storm Drain