Instructions
Visit Verdanturf Town, which is to the west of Mauville City. Enter the house at the bottom left of town. Speak to the girl with red hair. She will tell you the happiness level of your Golbat. If she says anything other than it's very happy or that it adores you, you need to increase its happiness.
Keep Golbat in your Pokemon party at all times. The simplest method to increasing Golbat's happiness is to have it in your party. Its happiness will increase as you walk and as he levels. Make sure it participates in various encounters so it will gain experience points.
Use all stat-boosting items you acquire on Golbat. Any item that increases health, attack, defense and other stats will also increase Golbat's happiness.
Revisit the happiness rater in Verdanturf Town. If she informs you that Golbat is very happy or adores you, focus on getting it to the next level. Once its level increases, it will evolve into Crobat.