Actually, I believe that all stage bosses are considered to be "trolls." If you look at the URL for each boss page (the URL inside the game's frame, not the Nutaku URL), they all have a query parameter called "id_troll" which is equal to a number. So for example, http://www.haremheroes.com/battle.html?id_troll=1 will take you to the Dark Lord, id_troll=2 is the Ninja Spy, id_troll=3 is Gruntt, and id_troll=4 is Edwarda. Don't ask how I know this :P

In short, all stage bosses, regardless of species, appear to be considered trolls. I would assume that the hint implies that girl drop rate on all stage bosses has been increased.