Here's what you should focus on to catch Registeel:
* Lower its HP: The lower Registeel's HP is, the easier it is to catch. Use your strongest Pokemon and attacks to damage it, but be careful not to faint it!
* Status Conditions: Inflicting a status condition like Sleep, Freeze, or Paralysis will further increase your chances of catching Registeel.
* Use a "Catch" Ball: Poke Balls are the most common, but Great Balls and Ultra Balls offer slightly better odds. You can also use a Master Ball to guarantee a catch, but this is usually saved for rare or legendary Pokemon.
* Increase Catch Rate: Some items, like the "Catch Rate" or "Dusk Ball" can help, but those are not guaranteed to work every time.
* Level Difference: The larger the level difference between your Pokemon and Registeel, the lower the catch rate will be.
Remember: Catching Pokemon is always a bit of a gamble, even with the best strategy. It's best to be prepared with multiple balls and a plan to deal with the possibility of it breaking free!