If your level is below 40 i'd recommend farming man juice from the 12 stamina gardens. 2-1 is a great map to give you said gardens and you'll receive a lot of coin too.

Basically, you'll want to get your girls to 30+ and after that start focusing on better manyu missions like the dailies. Save up lvl 5 manyus and throw 10 at a time on lower than 30 girls and throw 10x lvl 20 manyus on those above level 30.

Once you're at that point, with the help of a strong friend perhaps, you should be able to clear the best manyu dailies as well as one of the 11th area maps and thus gain access to 80 stamina gardens (skip the 50 gardens, they are not worth it for what they give).

Eventually you want to reach the 12th area where you have a chance to get ampys from mission chests in addition to the 80 gardens.