Aside from echoing what the above guys said, Krookodile was good when Aegislash was a thing in OU, but now it's not. Heatran is the better option over it, as not only does it resist Ice, but it also gets Stealth Rock up for you:
-Lava Plume
-Toxic
-Taunt/Protect/Roar
-Stealth Rock
Nature: Calm
EVs: 248 HP/192 SDef/68 Spe
Item: Leftovers
or
-Fire Blast/Magma Storm
-Flash Cannon/Stone Edge/Hidden Power (Ice)
-Earth Power
-Stealth Rock
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Air Balloon
or (the Magma Storm lure that Zekrom mentioned):
-Magma Storm
-Earth Power
-Taunt
-Toxic/Solar Beam
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Leftovers/Air Balloon/Power Herb
One major problem with your team is that 5 of the 6 Pokemon are weak to Ice, and the one that isn't doesn't resist it. Anything with an Ice attack will give you a hard time, like Mamoswine, Weavile, Black Kyurem, etc.
I think that the first thing to do would be to replace Crobat with Starmie. Crobat's not a bad poke, but Starmie gives you an Ice resist while still being able to remove entry hazards and resisting Fighting. And it resists Psychic. Water's just a good type in general:
The first set is a more defensive one that lets Starmie deal with Bisharp and Ferrothorn (and a few other things), common checks to Starmie, while the second is a straight-up offensive set if you want more firepower.
Then there's:
>Specs Hydreigon
>has Acrobatics
On a Specs set you'll generally wanna run all special moves. If you're looking for a mixed set, Life Orb or Expert Belt suit that better since mixed sets want the ability to switch moves. Hydreigon also has much better coverage moves than Acrobatics (which does not work well when holding an item, especially without STAB) like Fire Blast, Earth Power, Superpower, etc.
Gengar is, even with bulk investment, still a frail 'mon. Curse rips out half of its HP, and the opponent can just KO it and then switch out to remove the curse. Clear Smog is a nice move, but it's a little weak, attackers that don't rely on set up don't care about it, and as Gengar's only attacking move, it's getting walled by every Steel and Poison type out there. A better way to do SubSplit would be like this:
Though personally I think that a plain offensive set is better:
Serperior is also a very threatening poke thanks to Contrary Leaf Storm, but is somewhat easily walled and needs coverage moves like Hidden Power Fire/Rock, and Giga Drain/Knock Off provide some nice healing or utility, respectively.
But those are just set fixes. In addition to the Ice thing, some other issues with the team are:
1) how are you dealing with Fairies?
2) how are you handling stall?
3) how are you combating very fast offensive teams?
The Fairy thing isn't a terribly huge problem I suppose since you have Gengar and Serperior can possibly steamroll them if need be.
But you don't really have anything for fast offensive teams. Serperior and offensive Starmie are good, yes, but against teams filled with stuff like Mega Metagross, Mega Manectric, Mega Lopunny, priority moves, and Scarfers, they aren't enough. Admittedly those Pokemon are hard to handle for more offensively inclined teams since those Pokemon are designed to break offense, but stuff like Prankster Thundurus-I, Klefki, Sableye, and your own priority moves help with that to some extent. Oh, and opposing Gengars are kinda threatening to the team but Gengar threatens a lot of teams no matter what you do so....
As for stall, you don't exactly have much either for that. Whimsicott can annoy stall but it's not exactly breaking it either and you'll probably eventually lose. Hydreigon does wallbreak (sorta), but that's still just one 'mon against an entire team. Some stallbreakers to consider are like.....Keldeo, Magma Storm+Taunt Heatran, Mega Gardevoir, Mega Charizard (both forms, I think).
Probably should replace Krookodile to make room for some of that stuff. Krookodile is great in UU, and Ground/Dark is a nice offensive STAB combo, but I think that after Crobat it's the most replaceable member on the team right now and you really imo need some stuff to handle stall and fast offense better. Removing it also makes for one less Water, Ice, Grass, and Fighting weakness.
jesus christ why the psyduck did I write so much
Destiny Bond is also an option in the last slot for Gengar.
If you're looking for better Hydreigon sets, here they are:
-Dark Pulse
-Draco Meteor
-Flash Cannon/Fire Blast
-Superpower/Earth Power
Nature: Mild/Rash/Modest
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
To further elaborate on the above users' points, sets for Thundurus, Klefki and (Mega) Sableye. To be honest, this team needs a Mega Evolution, IMO (this is all done over Whimsicott):
Thundurus:
-Thunderbolt
-Hidden Power (Ice)
-Focus Blast/Psychic/Taunt
-Thunder Wave/Taunt
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Leftovers/Life Orb
Ability: Prankster
or
-Nasty Plot
-Thunderbolt
-Hidden Power (Ice/Flying)
-Thunder Wave/Focus Blast/Substitute
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb/Leftovers
Ability: Prankster
or
-Thunderbolt
-Hidden Power (Ice)
-Knock Off
-Superpower/Thunder Wave
Nature: Naive
EVs: 76 Atk/180 SAtk/252 Spe
Item: Life Orb
Klefki:
-Spikes
-Thunder Wave
-Toxic/Magnet Rise
-Play Rough/Foul Play
Nature: Careful/Calm
EVs: 252 HP/4 Def/252 SDef
Item: Leftovers
Ability: Prankster
Sableye:
-Knock Off
-Will-o-Wisp
-Recover
-Foul Play/Toxic/Metal Burst
Nature: Careful
EVs: 252 HP/112 Def/144 SDef
Item: Sablenite
Ability: Prankster
or
-Calm Mind
-Shadow Ball/Dark Pulse
-Will-o-Wisp
-Recover
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Sablenite
Ability: Prankster
Speedy offensive Mega Evolutions you can use on the team include Metagross and Lopunny:
Metagross:
-Meteor Mash
-Zen Headbutt
-Ice Punch/Bullet Punch/Grass Knot
-Earthquake/Hammer Arm
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Metagrossite
Ability: Clear Body
or
-Agility
-Meteor Mash
-Zen Headbutt
-Earthquake/Hammer Arm
Nature: Adamant
EVs: 80 HP/252 Atk/176 Spe
Item: Metagrossite
Ability: Clear Body
Lopunny:
-Return
-High Jump Kick
-Fake Out
-Ice Punch/Power-Up Punch
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Lopunnite
Ability: Limber
or
-Return
-High Jump Kick
-Substitute
-Baton Pass/Encore
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Lopunnite
Ability: Limber