If you're playing Diamond, you can use other moves to lower an opponent's HP to a manageable level before capturing it, such as:
* Scratch: A basic move that does low damage.
* Tackle: Another basic move that does low damage.
* Growl: Lowers the opponent's Attack stat.
* Leer: Lowers the opponent's Defense stat.
* Thunder Wave: Paralyzes the opponent, making it less likely to attack and allowing you to land more attacks.
You can also use a Pokemon with the ability Pickup, who can find a Rare Candy while exploring. This will allow you to increase the level of your Pokemon, thus lowering the chance of them fainting during battle.