What this song renders
The Genpei War (1180–1185) was the civil war between the Minamoto (Genji) and Taira (Heike) clans that ended the Heian aristocratic order and produced the Kamakura shogunate. Yoshinaka commanded the western branch of the Minamoto cause — descended from a dispossessed line of the family, raised in the mountainous Kiso region, fighting his way south and east toward Kyoto. By summer 1183 he had taken the capital and forced the Taira to flee with the child-emperor.
The Heike Monogatari’s description of Tomoe in Book 9 is the only contemporary indication of her battlefield record. The famous line — a swordswoman worth a thousand, ready to confront a demon or a god — is a literary formula, but it is the kind of formula the Heike uses for its most-recognised warriors. It is praise of a specific kind, not a general accolade.
The song does not render any single named engagement. Yoshinaka’s campaign included encounters at Yokotagahara, Hiuchi, and Kurikara before he reached Kyoto; Tomoe is not named in any of them by the surviving sources. What the song renders is the kind of fighting the Heike’s description implies — the demonstration that earned the line.
The general arc — Tomoe present, Tomoe lethal, Tomoe at the head of Yoshinaka’s column — is consistent with the Heike. No specific battle is anchored to her here. The song is the kind of fighting the Heike says she did, set to its largest sound.