Prepared and Spontaneous Spells
Source GM Core pg. 126
Spell slots work best for creatures that are meant to play like PC spellcasters. Choose the magical tradition best suited to the creature. You aren't strictly limited to that tradition's spell list, though sticking close to it will make your creature's connection to that tradition clearer. The decision to use prepared or spontaneous spellcasting should align with the creature's theme: a spontaneous spellcaster fits well as a one-off creature since spontaneous spellcasting grants greater flexibility in the middle of battle, while a prepared spellcaster makes for a great recurring character who can change their spells between appearances.
For a creature that can cast as many spells as a PC spellcaster, the highest spell rank the creature can cast is half its level rounded up. It gets five cantrips. If the creature's level is odd, it gets two spell slots of the highest spell rank (plus three spell slots of each lower rank), or three spell slots of that rank (plus four spell slots of each lower level). If its level is even, it gets three spell slots of the highest spell rank (plus three spell slots of each lower rank), or four spell slots of that rank (plus four spell slots of each lower rank). You can base the number of spells on the class you're trying to emulate or choose more spells if the creature doesn't have many other abilities.
Because creatures tend to be “on stage” for only a short time, you usually don't need to fill every spell slot. You can often fill just the top three ranks of spells, pick cantrips, and slot in a few thematic backup spells in the fourth rank down. For a recurring foe, you might give it a full complement of spells.
For a creature that can cast as many spells as a PC spellcaster, the highest spell rank the creature can cast is half its level rounded up. It gets five cantrips. If the creature's level is odd, it gets two spell slots of the highest spell rank (plus three spell slots of each lower rank), or three spell slots of that rank (plus four spell slots of each lower level). If its level is even, it gets three spell slots of the highest spell rank (plus three spell slots of each lower rank), or four spell slots of that rank (plus four spell slots of each lower rank). You can base the number of spells on the class you're trying to emulate or choose more spells if the creature doesn't have many other abilities.
Because creatures tend to be “on stage” for only a short time, you usually don't need to fill every spell slot. You can often fill just the top three ranks of spells, pick cantrips, and slot in a few thematic backup spells in the fourth rank down. For a recurring foe, you might give it a full complement of spells.