For EVs, it's better if your read
this.
For IVs, read
this
Put Gigalith in the lead slot. Replace Explosion with Earthquake and make it Adamant. Make its EVs 252 HP/252 Atk/4 Def
Don't use Special attacks on Darmanitan, unless your using its novelty Zen Mode form because of its poor Special Attack. Belly Drum is also too situational and risky, IMO. Something like this hits a
lot harder:
-Flare Blitz
-Rock Slide
-Earthquake/Superpower
-U-turn/Fire Punch
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Choice Band/Choice Scarf
Ability: Sheer Force
Just so you can make use of Dragonite's Hasty nature, you can use Fire Blast in place of Fire Punch. Make its EVs 4 HP/252 Atk/252 Spe.
Swampert doesn't do too well as well as it used to. I think Jellicent may be a better wall:
-Scald/Surf
-Toxic/Will-o-Wisp
-Recover
-Ice Beam/Taunt/Shadow Ball
Nature: Calm/Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Water Absorb
Give Scizor Superpower or Brick Break so that it has a way of dealing with Steels. Make its EVs 4 HP/252 Atk/252 Spe.
You're not taking advantage of Breloom's Poison Heal at all:
-Substitute
-Focus Punch
-Spore
-Seed Bomb/Stone Edge
Nature: Jolly
EVs: 12 HP/252 Atk/244 Spe
Item: Toxic Orb
Ability: Poison Heal