Instructions
Raise Golbat's happiness by giving him vitamins, such as Iron, using him in Pokemon battles, gaining levels with him and walking around with him in your Pokemon lineup.
Travel to Goldenrod City.
Enter the house to the right of the Goldenrod Department Store.
Speak to the woman inside with Golbat in the first position of your Pokemon lineup. If his happiness is at its maximum, she will tell you that "It looks really happy! It must love you a lot." If you receive any message other than this, continue to raise Golbat's happiness until you receive this message.
Battle wild Pokemon and other trainers with Golbat until he earns enough experience to gain a new level. The amount of experience Golbat requires is located underneath his health bar in battle. When Golbat gains a level and is at maximum happiness, he will evolve into Crobat at the end of the battle.