This text is a translation of a Middle Age version of Tristan et Iseult written originally in Norman language in the 12th century by Béroul. So you must expect to encounter lots of vocabulary issues even if it is supposed to be a translation into “modern French”. The problem with this translation is that although it is supposed to be in modern French the translator has used words that have changed meanings nowadays. It must be very confusing for someone who does not master the language sufficiently. This translation is usually used in French secondary schools for native French students.

requérir de l’épée
Here the verb requiert is used in the sense it had in the Middle Ages. In a warfare context requerre meant “to choose your adversary in order to attack him”. So the sentence just means that Tristan attacked the dragon (le) once more with his sword.

From the Dictionnaire historique de la langue française

Requerre s’employait en ancien français pour « prier (qqn), demander (qqch.) » [v. 980], réclamer (qqch.) » [v. 1050] et, spécialement dans un contexte guerrier « choisir (son adversaire) pour l’attaquer » et « attaquer » (1080), emplois disparus.