Originally Posted by Drip

Skill blooms, especially the Rainbow Skillbloom (500 crystals), but if you don't have a rainbow girl yet, and don't want to stock up on these skillblooms in advance, then gold skillblooms (250 crystals a piece) are a good investment as well. It's easier to raise the skill through skillblooms (guaranteed skill-up), than by merging with a copy of the same girl (harder to obtain, and you're not guaranteed to get a skill-up).

No need to use skill blooms on girls that were event rewards. It's better to use skill blooms on girls you obtained from Gacha.

This is the only way to get petals right now, so have 300 spare at all times if you want to bloom girls. The 150 heart isn't horrible, but there are other ways to get them.