Banpresto published quite a few licensed games based on popular anime series in the 90s, and the rather plainly named Slayers was one of them, a relatively generic comedy RPG about an incredibly powerful (and incredibly flat chested) mage named Lina Inverse. The game was developed by AIC Spirits (per GameFaqs) and published by Banpresto, which had done a number of other popular licenses, all of which show a fair amount of polish. The game itself is a pseudo-3D dungeon crawler with turn-based combat of a rather standard nature. As Lina starts the game a powerful mage, you don't level up in this one, but you do get Gourry and up to two additional characters.

There is now a English fan-translation of this game