You definitely need a Stealth Rock user on this team. While I think your Celebi set is innovative, it is total set up bait. Anything with Taunt is going to shut it down completely, Gyarados in particular. I would change its moveset to:
Celebi @ Leftovers
Ability: Natural Cure
EVs: 244 HP / 220 Def / 44 Spe
Nature: Bold (+Def, -Atk)
Leech Seed | Grass Knot | Recover | Stealth Rock
Leech Seed forces a lot of switches. Bulky Waters (Suicune and Vaporeon) and Blissey, easily wall Empoleon. Leech Seed will force them to switch out and it takes advantage of their high HP. Grass Knot also does decent damage on Suicune and turns Celebi into a great Gyarados counter. Stealth Rock should be mandatory on any team. Recover keeps Celebi alive for longer.
Like others have suggested, I would replace Dusknoir with Rotom-H. It may not have a priority move like Dusknoir, but it has much better typing as well as an immunity to Ground attacks.
Rotom-H @ Choice Scarf
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
Nature: Timid (+Spe, -Atk)
Discharge | Shadow Ball | Overheat | Trick
This Rotom is an amazing revenge killer, allowing you to kill Gyarados and Lucario with the appropriate move. Discharge over Thunderbolt for the higher paralysis chance, which is always nice. Trick is amazing for this team because it allows you to cripple Blissey, allowing Empoleon to sweep much easier.
Celebi @ Leftovers
Ability: Natural Cure
EVs: 244 HP / 220 Def / 44 Spe
Nature: Bold (+Def, -Atk)
Leech Seed | Grass Knot | Recover | Stealth Rock
Leech Seed forces a lot of switches. Bulky Waters (Suicune and Vaporeon) and Blissey, easily wall Empoleon. Leech Seed will force them to switch out and it takes advantage of their high HP. Grass Knot also does decent damage on Suicune and turns Celebi into a great Gyarados counter. Stealth Rock should be mandatory on any team. Recover keeps Celebi alive for longer.
Like others have suggested, I would replace Dusknoir with Rotom-H. It may not have a priority move like Dusknoir, but it has much better typing as well as an immunity to Ground attacks.
Rotom-H @ Choice Scarf
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
Nature: Timid (+Spe, -Atk)
Discharge | Shadow Ball | Overheat | Trick
This Rotom is an amazing revenge killer, allowing you to kill Gyarados and Lucario with the appropriate move. Discharge over Thunderbolt for the higher paralysis chance, which is always nice. Trick is amazing for this team because it allows you to cripple Blissey, allowing Empoleon to sweep much easier.