I would get rid of Spikes because poisoning them would be better. Stealth rock is for flying types. For EVs I would give him the following 252 HP / 128 Def / 128 SDef. To replace Spikes I would give him Volt Switch for priority.
I would keep the EV's you gave him but give him those four leftover ones to Special Defense. Change Hydro Pump to Surf because it has better accuracy, even though it has less Base Power. I would definitely get rid of Giga Impact seeing as it's a recharge move, I would change it to Thrash. Give him the Persim or Lum Berry to get rid of confusion. Dive isn't too good seeing as you already have a water type move. Change that to Earthquake to deal with Rock and Electric types. If you have the time of think your opponent is going to switch I would recommend Dragon Dance.
Oh Gengar, what a great Pokemon. The item for him should definitely be black sludge to recover HP. Give him four EVs in Special Defense. Change Dark Pulse to Focus Blast so you can cover Dark types. Gengar has horrible Attack, what are you doing, change that into Giga Drain to regain HP. Shadow Ball is fine. Change Night Shade into Sludge Bomb for poison hax and for STAB.
The moves are fine except for Draco Meteor. Haxorus is better physically. I would change that to Earthquake, in case an Electric type comes in to get your Gyarados. I would change the 252 HP EVs to Speed EVs so he can hit Electric types faster.
I would really change her Egg Bomb attack to Seismic Toss to reasure you that you will be doing damage. Your Blissey is fine other wise.
252 Speed EVs would be better than HP if it's a sweeper. Change Blizzard to Ice Beam for a more reliable hit. Rapid Spin is good to get rid of hazards. I would change Hydro Pump to Thunderbolt, because more than one of your type isn't necessary.