The stack has been set correctly and the drop amount, too, and yet it is not working as intended?
Might be just another Joymax bullcrap. For instance, you cannot declare a drop with +8 or higher, even while Joymax had a plus drop value set as 12 max. I tried it. Didn't even matter setting degreed item rates to 100% of each level assignment and plus chance range, +7 was the max.
Welp, about the Roc stuff, Joymax literally copied lots of stuff from other games, since the European race got announced and released. You will find, as a pretty passionate Lineage 2 player, a lot of similarities to the stuff, which declares to be the European expansion. Tanks, wizards, rogues, etc. (oddly, summoners are missing, lol), as well the aggro pull and trigger system (attacking a crowd will attract them all with a high amount of aggro puffer, which only decays after a while or by debuffing/someone else damaging), are bloody close to L2.
Roc on it's own is more a "Raid/Instance Boss", instead of a simple roaming Boss, or as we call "Unique". Raid bosses are mainly being fought in a group and shouldn't be soloable, and the amount should be shared among all anticipating players/parties/clans. Should be the same for Medusa, too.
Oh well. We won't ever get smart of Joymax's work. They are even declaring regionID to a limited smallint, who the fuck messed that up? Wanna teleport your character to a region grid higher than 32,767? Or wanna add regions like the Jupiter Temple's zone B and C instances, or those 3 Jupiter unique rooms as to their own origin coordinate? Well, nope. You cannot, but you can walk and exit there, and yet the server module sends the correct information to saves em.