Farming wise 2-1 or 1-3 are pretty much on par (1-3 is cheaper but less gold and seeds), and both can trigger the bonus stage. Keep on farming until you have a bunch of girls evolved and levelled up again to 20s or 30s, at which point upgrading will be costly. Until you can clear 12-3 & 12-5 (and the bonus stage from them), just stick with 2-1 or 1-3.

Also, what Miu posted as well. 4*s are not that hard to evolve. Don't be afraid to upgrade or even evolve your 3*s too. There are even easier to evolve, and have more possibility of you maxing their skill levels & equipment slots.

When you reach level 40, start upgrading your equipment too. Upgraded rings can scale quite nicely.