Use a Pokémon that can take advantage of Thundurus's weaknesses. Thundurus is weak to Ice, Rock, and Ground-type moves. If you have a Pokémon that can use one of these types of moves, it's a good idea to use it.
Use a Pokémon with a high Special Defense stat. Thundurus's Thunder attack is a special move, so it's important to use a Pokémon with a high Special Defense stat. For example, you could use a Pokémon like Blissey or Chansey.
Use a Pokémon with the ability "Lightning Rod". The ability "Lightning Rod" allows a Pokémon to redirect Electric-type moves away from its allies and to itself. This can be very useful against Thundurus, as it will prevent it from damaging your other Pokémon. For example, you could use a Pokémon like Alolan Raichu or Luxray.
Use a Pokémon with the ability "Thunder Wave". The ability "Thunder Wave" allows a Pokémon to paralyze its opponent. This can be very useful against Thundurus, as it will make it much harder for it to move and attack. For example, you could use a Pokémon like Jolteon or Manectric.
Use a Net Ball. A net ball is a type of Poké Ball that is more likely to catch a Pokémon that is flying. Thundurus is a Flying-type Pokémon, so it's a good idea to use a Net Ball to catch it.
Use a Quick Ball. A quick ball is a type of Poké Ball that is more likely to catch a Pokémon at the beginning of battle. Thundurus is a powerful Pokémon, so it's a good idea to use a Quick Ball to catch it before it has a chance to attack.
Use a Dusk Ball. A dusk ball is a type of Poké Ball that is more likely to catch a Pokémon at night. Thundurus is a Pokémon that can only be found in the Sinnoh region, and the Sinnoh region is dark at night, so it's a good idea to use a Dusk Ball to catch it.