Pokemon Showdown Gen 8 National Dex

Question

Talonflame (M) @ Sharp Beak
Ability: Flame Body
EVs: 252 Atk / 4 SpA / 252 Spe
Jolly Nature
- Brave Bird
- Roost
- Flame Charge
- Tailwind

Blissey @ Leftovers
Ability: Healer
EVs: 252 HP / 4 SpA / 252 SpD
Bold Nature
IVs: 0 Atk
- Soft-Boiled
- Dazzling Gleam
- Toxic
- Focus Blast

Gyarados-Mega (M) @ Gyaradosite
Ability: Moxie
Shiny: Yes
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Crunch
- Waterfall
- Dragon Dance
- Avalanche

Magnezone @ Magnet
Ability: Analytic
Shiny: Yes
EVs: 252 HP / 252 SpA / 4 Spe
Modest Nature
- Explosion
- Thunderbolt
- Rest
- Steel Beam

Donphan (M) @ Smooth Rock
Ability: Sand Veil
Happiness: 0
EVs: 4 HP / 252 Atk / 252 Def
Adamant Nature
- Sandstorm
- Earthquake
- Frustration
- Double-Edge

Salamence (M) @ Dragon Fang
Ability: Moxie
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Dragon Dance
- Stone Edge
- Flamethrower
- Outrage

Comments

A few of your move and Pokémon choices are quite questionable.

For starters, Talonflame sucks if it's not ORAS, and even then you need a Tran answer.

2. Change Blissey to Heavy-Duty Boots and Natural Cure than Leftovers and Healer respectively. Change Moveset to:
- Soft-Boiled
- Teleport
- Toxic
- Seismic Toss

3. Change Gyarados's ability to Intimidate. This'd let it setup on foes that'd otherwise make its life hell (namely physical attackers who don't like Intimidate). Change Avalanche to Substitute maybe.

4. Magnezone is bad. Do
Magnezone @ Choice Specs
Ability: Magnet Pull
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Thunderbolt
- Volt Switch
- Hidden Power (Fire)
- Flash Cannon

I don't play NDOU but this is better than what you run atually.

5. Bad. Do Garchomp.
6. Bad. Do Greninja-Ash.

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Magnezone isn’t bad, but I don’t really see how it fits well on this team.  If you replace talonflame with heatran (and you absolutely should) magnezone isn’t that important on this team and thus should be replaced. Considering this team needs a lot of work and the final product will probably look like an offense team, Blissey should be replaced entirely.