The best way I found to evolve your budsquire into Melloblum is to:
- Level Budsquire to level 25
- Complete the quest "Flowers and Thorns" (to unlock the evolution)
- Have a personality that contains "Considerate"
- Acquire "Stamen of Cubbo's Rose"
How to get Stamen of Cubbo's Rose:
- Twine with Budsquire and basic attack the flower. It has a low chance to drop. There will be a purple glitter transparent sphere that drops, run over it to pick it up.
The flowers are guarded by Cubbo, so be careful!
Hopefully this helps!