The King's Avatar movie is a pretty enjoyable movie to watch. However, as a standalone movie, the shortcomings become to show, particularly in the story and character department. Art: The art is beautiful, on par with some great Japanese anime. The animation, aside from the hit or miss parts before the final climax, is excellent as well. The final fight between the MC and the last opponent, especially, was godly. Sound: The OST is pretty good as well. Especially on the final fight with, combined with the animation, makes it a great experience to watch. Story: Now this is where the flaws begin. This story serves as aprequel before the entire main series. However, even then, the plot was rushed, especially since you're cramming a prequel novel into a 90 minutes movie. There's seem to be pretty rushed directing as well, since between the time of the team forming to the finals except for one point in between, it's just going to places to places. It's sufficient as a companion piece, but as a standalone, it fell short

Character: Likewise, because of how rushed the story was, the character development were rushed as well. Aside from MC, his best friend and the sister, the other characters' development were lacking. We didn't know much about them aside from the Spellblade guy.

Overall: Despite the shortcomings in the story and character department, plus some hit-or-miss animations before the finals, it's a very enjoyable movie to watch to fill in the time while waiting for Season 2.

My recommendation is watch at least Season 1 (and maybe the OVA) before jumping onto the movie.