EVs, abilities, items and natures please? Natures help one stat and hinder the other. For example, a Modest nature boosts Special Attack slightly, but lowers Attack slightly.
This helps explain the importance of natures. To elaborate, natures boost one stat slightly while hindering another. This is useful for Pokémon who focus solely on one offensive stat; on mixed sweepers, it's a bit tricky, as they may have to lose out slightly on a defensive stat so that they have extra power or Speed. On defenders, it helps them take hits a bit more easily on one defensive side. For example, Hydreigon is primarily seen as a special or mixed attacker, so a Modest (boosts Special Attack, but lowers Attack; preferred for pure Special Hydreigon) Mild or Rash (both boost Special Attack, but the former lowers Defense, while the latter lowers Special Defense; preferred for mixed variants) nature is preferred
You also need to specify your 'mons' Hidden Power types.
Hydreigon currently has nothing for it to take on Steels. Its movepool is extremely wide as well:
-Draco Meteor
-Fire Blast
-Superpower
-Earthquake/Earth Power/Dark Pulse/Roost
Nature: Rash/Mild
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
or
-Substitute
-Dragon Pulse
-Focus Blast
-Fire Blast/Roost
Nature: Modest
EVs: 4 HP/252 SAtk/252 Spe
Item: Leftovers/Life Orb
or
-Draco Meteor
-Fire Blast
-Focus Blast
-U-turn/Dark Pulse
Nature: Modest
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Specs
I'll assume that the Venusaur you got is the Overgrow variant. Without Chlorophyll (Venusaur's Dream World ability) and Drought support from Ninetales, it's seriously mediocre. Ferrothorn is a better option because of its high defenses and good typing:
-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
You do realize that Braviary has Superpower to hit Steels hard, right?:
-Return
-Brave Bird
-Superpower
-U-turn/Roost (latter Life Orb only)
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band/Choice Scarf/Life Orb
Ability: Defiant
Chandelure's Hidden Power type should be Fighting. A Modest nature should be used with the EV spread of 120 HP/252 SAtk/8 SDef/128 Spe. And you can just evolve Lampent into Chandelure as soon as you get it, then use the Shadow Ball TM on it (it's right behind the Strange House).
Samurott can force switches, but it's better off setting up Swords Dance or hitting the switch-in with the appropriate coverage move. Air Slash is unnecessary on it, IMO:
-Swords Dance
-Waterfall
-Aqua Jet
-Megahorn
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Lum Berry/Mystic Water
or
-Surf/Hydro Pump
-Ice Beam
-Grass Knot/Hidden Power (Grass)
-Taunt/Megahorn/Superpower
Nature: Timid/Naive
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
Hyper Beam style moves are a big no-no, no matter how tempting the base power is (especially on Metagross). They leave the user a sitting duck the next turn:
-Stealth Rock
-Meteor Mash
-Pursuit
-Earthquake/Hammer Arm
Nature: Adamant
EVs: 252 HP/96 Atk/160 SDef
Item: Leftovers
or
-Meteor Mash
-Bullet Punch
-Hammer Arm
-Zen Headbutt/Pursuit/ThunderPunch/Ice Punch
Nature: Adamant
EVs: 164 HP/252 Atk/92 Spe
Item: Choice Band
or
-Agility
-Zen Headbutt
-Earthquake
-Meteor Mash/Ice Punch
Nature: Jolly/Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb
or
-Meteor Mash
-Hidden Power (Fire)
-Grass Knot
-Earthquake/Zen Headbutt
Nature: Naughty
EVs: 252 HP/228 SAtk/28 Spe
Item: Life Orb
To clear out your Pokémon's EVs, use the Salon in Join Avenue, assuming your Pokémon's happiness isn't full, and you hired the right person.