Solasta: Crown of the Magister

After each fight with an enemy you do not yet have 4/4 bestiary info on, the most appropriate character will roll a Survival check to see if they gain info on that enemy type. If it succeeds, 1/4 info will be added to the bestiary, and this achievement will progress. This will naturally unlock over regular play as long as you have a character with a moderate Survival score.

Keep in mind that there is a "cheat" option in gameplay settings that can enable all bestiary to be unlocked from the get-go. You must NOT play with this on in order to unlock the achievement. Repeat: do NOT turn on the bestiary cheat.