{Alchemist} Gorgath
[a variant of this NPC, found near Annolisa Rosie, at The Great Library of Calpheon (SE section from main city icon of the map) or search Annolisa Rosie and travel to her, he's by the table.]

Caveat: he may not be there or interactable until you reach a certain point in the main questline could require Calpheon to be completed or Abyssal Welliportation questline, my alt just finished Mediah and started Valencia.

WHY THE PREAMBLE WAS IMPORTANT
some knowledge may also be quest locked so do it on the character with most quest progress JIC
*******Alchemist Gorgath offer's free BOSS KNOWLEDGE via conversation.*******

CHAT BOSS KNOWLEDGE

- Biraghi
- Queen Stoneback Crab
- Gehaku
- Kelcas
- Ruins Guard Tower
- Org

(there may be more, and I apologize for not having a comprehensive list, but I have S-Grade Knowledge for some Bosses and am not willing to delete it, So I will rely on the community to comment on any others they were able to get =)

on that note: reference NPC Annolisa Rosie for details on how to S proc Boss Knowledge link below should bring you to her BDO Codex page