Arcanine's defenses aren't necessarily the best, but it is outsped by other common Pokémon, so a Pokémon with Sticky Web could be useful. If you want to keep SR, then a good option would be Shuckle. It's defenses make it difficult to get off the field and a standard stall set could suffice. If you don't mind getting rid of SR then Galvantula would work.
Shuckle @ Leftovers
Ability: Sturdy
Tera Type: Fairy
EVs: 252 HP / 60 Def / 196 SpD
Bold Nature
IVs: 0 Atk / 0 Spe
- Sticky Web
- Stealth Rock
- Protect
- Toxic / Infestation
Standard support for slower allies like Arcanine. Toxic let's it deal good damage while it tanks hits.
Galvantula @ Focus Sash
Ability: Compound Eyes
Tera Type: Water
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Sticky Web
- Thunder
- Energy Ball
- Volt Switch
This set lets Galvantula provide support and deal respectable damage. Thunder's accuracy is increased via Compound Eyes, Energy Ball kills off Ground types and Volt Switch is for a fast pivot.
Now for the new Arcanine.
Arcanine @ Choice Band
Ability: Intimidate
Tera Type: Electric
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Flare Blitz
- Extreme Speed
- Wild Charge
- Play Rough
A Choice Band Wallbreaker set. Flare Blitz for STAB, Play Rough for Dragons, Wild Charge for bulky Water types like Vaporeon, and E-Speed for revenge killing.
As Gliscor is now banned in NDUU (thanks @-RisingManectric- for pointing that out), a replacement is required. I would recommend Mamoswine. It can deal good damage and has respectable bulk. It works well as a bulky Wallbreaker with max attack and Choice Band or a fast sweeper with Choice Band Ice Shard or Choice Scarf.
Wallbreaker/Bulky Attacker
Mamoswine @ Choice Band
Ability: Oblivious
Tera Type: Ice
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Avalanche
- Ice Shard
- Earthquake
- Stone Edge
Bulky attacker with Avalanche for high damage and Ice Shard for frail opponents.
Fast Attacker
Mamoswine @ Choice Scarf
Ability: Oblivious
Tera Type: Ice
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Earthquake
- Icicle Crash
- Knock Off
- Stone Edge
Offensive Scarf attacker with 2 STAB and 2 coverage.
For Quagsire, swap out Surf for Recover. This helps improve survivability and you still have Earth Power for STAB with Ice Beam for Flying types.
I don't see anything wrong with Gardevoir.
For a second sweeper, I have three options for you: Alakazam, Haxorus or Blacephalon.
Alakazam @ Life Orb
Ability: Magic Guard
Tera Type: Dark
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Nasty Plot
- Psychic
- Dazzling Gleam
- Shadow Ball
All moves are buffed by Life Orb with no drawbacks thanks to Magic Guard. NP for set-up, Psychic for STAB and the other two for coverage.
Haxorus @ Muscle Band
Ability: Mold Breaker
Tera Type: Fairy
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Close Combat
- Dragon Dance
- Dragon Claw
- Iron Head
Standard set-up sweeper. 2 coverage moves and 1 STAB.
Blacephalon @ Choice Specs
Ability: Beast Boost
Tera Type: Grass
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Fire Blast
- Shadow Ball
- Psyshock
- Hidden Power [Grass]
Fast specs with powerful STAB and coverage.
Out of the 3, I recommend Haxorus the most.