You can watch the English dub of Seirei Gensouki: Spirit Chronicles (Season 1 and Season 2) Episode 12 for free by using official streaming service trials. While Crunchyroll has largely ended its general ad-supported free tier, several authorized platforms still offer free access windows. Official Free Viewing Methods
Depending on which season you are looking for, Episode 12 serves as a major finale: Watch Seirei Gensouki (Dub) Episode 12 for free...
Watch the official Season 2 finale (English Dub) directly on Crunchyroll: You can watch the English dub of Seirei