Best strat for upgrading new girls as follows:

Feed 10 x lvl 5 manyu twice you should be around lvl 29-31.
Feed 10 x lvl 20 manyu after that and if possible save lvl 100s for girls over lvl 50.

That is the easiest way to level and not that bad gold wise either.
Of course the most gold efficient is feeding 10 x lvl 100 each time till max level but you don;t get that many lvl 100s easy.