At the moment, only certain girls have abilities for fighting the world bosses. They are Desert guard, Jungle queen, Black mage, Yakuza, Venus, Sheela na gig from the Country cousins set, and Sorcerer from the Dungeon crawlers set (from the latest event).
If you click on the girl, then Ability at bottom of list (at the right), you can see the cost to upgrade their ability.
To upgrade to ability level 2, you would need to level up the girl to level 5, and have 5 libido stones of the correct colour / element. To reach level 3 of the ability, you need the girl at level 10 and 10 libido stones of same element / colour.