First, both of these classes should have every creature in the game included in their Hunting Logs, and their logs should be organized by monster type, then level.

Second, in order to unlock a specific creature/ability, the BST/BLU must defeat one of each monster of that type, up to the level of the creature/ability he's trying to unlock.

For a BLU example, to learn "Screwdriver" might only require besting a Pugil at level 5, but to get "Sanguine Bite" the character would have to defeat a Lv 10 Dirty Mongrel, Lv 15 Alpha Wolf, and a Lv 20 Wild Wolf.