Runerigus (M) @ Leftovers  
Level: 72
Ability: Wandering Spirit  
- Poltergeist  
- Earthquake  
- Will-O-Wisp  
- Stealth Rock  

Volcarona (M) @ Focus Sash 
Level: 72
Ability: Flame Body  
- Quiver Dance  
- Fiery Dance  
- Bug Buzz  
- Scale Breeze 

Melmetal @ Assault Vest  
Level: 73
Ability: Iron Fist  
- Double Iron Bash  
- Ice Punch  
- Thunder Punch  
- Earthquake  

Coalossal (M) @ Power Herb 
Level: 73
Ability: Steam Engine  
Gigantamax: Yes  
- Meteor Beam  
- Overheat  
- Scald  
- Scorching Sands  

Garchomp-Mega (M) @ Garchompite  
Level: 72
Ability: Rough Skin  
- Swords Dance  
- Scale Shot  
- Earthquake  
- Stone Edge  

Eternatus @ Dragonium Z  
Level: 74
Ability: Pressure  
- Dynamax Cannon  
- Sludge Wave  
- Flamethrower  
- Eternabeam  

Team Ultra Galaxy Leader Chairman Westing is my toughest fan fiction antagonist team leader by far, not just because he actually has a Legendary but moreso because his strategy is borderline competitive. His Runerigus can set up Stealth Rock, some of his Pokemon have powering up moves, and his Eternatus can unleash a 200 base power Z move with Eternabeam!

However, this is not to say he is impossible to beat. Runerigus's set up set leaves Chairman Westing extremely open to your own power up moves such as Gyarados's Dragon Dance. Giratina is useful here as are the Sinnoh starters, especially Empoleon which beats half of Chairman Westing's team in type matchups; it only needs Ice Beam or Blizzard to deal with the dragons. Melmetal is tough to beat because its defenses are really stocky and it's moveset is somewhat varied but if you have a strong Fighting type it should work decently. The real threat of the team is Eternatus, and trying to use Giratina or other dragons on it is really risky. Instead, try using Heatran (which is resistant or immune to all of Eternatus's moves) and use Dragon Pulse or Earth Power, or Attack Deoxys (preferably an overleveled one with Psycho Boost + Life Orb/Psychium Z).