There's no level requirement, it evolves by Happiness. It could evolve as soon as Level 2 or as late as say Level 98, it all depends on how happy you make it be. The Soothe Bell is a good first step but other ideas include giving it vitamins that taste good, giving it massages, walking around with it and battling with it and making sure you win with it and never let it faint. Vitamins that are helpful for Happiness include mainly Stat raising ones like Calcium, Iron, Carbos, HP Up, Protein and Zinc. The massage people are in Veilstone City and in the Resort Area but in order to get into the Ribbon Society in the Resort Area, I believe you need to have a Pokémon with you that has 10 Ribbons in total. It doesn't matter where the Ribbons came from as long as there's 10 total.
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Level it up with max happiness. You can give it a soothe bell, and train it.
make it hold soothe bell until it reaches level 36
Roselia doesn't necessarily need a soothe bell to evolve, but yes it would increase the happiness level of Roselia which is one of the requirements. The additional requirement for Roselia to evolve is that you must give it a shiny stone during the day.
You give on of your Pokemon the soothe bell and it will get happy faster.(It helped me evolve my eevee and togepi.)
Golbat can evolve in any level. Golbat evolves into Crobat via happiness. Your Golbat must be happy enough until it will evolve. It can evolve from having fun with battles. I suggest you give it a Soothe Bell to make this proggress much quicker.