Annoyed that Megumin had still been using his castle for explosion practice, Beldia, after proving easily susceptible to Aqua's purification magic, summons an undead army which is quickly defeated by Megumin's explosion magic. As Beldia then battles against Darkness, Kazuma deduces that he is weak against water and has Aqua attack him with a flood. Managing to weaken him, Kazuma manages to steal Beldia's head, giving Aqua the chance to defeat him with her magic. The next day, Kazuma and his party earn a hefty reward for defeating Beldia, which is immediately put towards paying off the damages Aqua's flood caused to the town.
In the Light Novel, the furious Beldia was able to stop himself before slamming his own head on the ground. In the Anime, Beldia got so mad at Kazuma's party, he slammed his head on the ground.
In the Light Novel, a group of Undead Knights accompanied Beldia to Axel. In the Anime, they were summoned with magic.
Megumin was exhausted after she used Explosion to take out all the Undead Knights' underlings. In the Light Novel, she tried to attack the Axel adventurers when they complimented her, but referred to her as "Crazy Girl." In the Anime Megumin just asks Kazuma to remember the adventurers faces, so she can take her revenge on them later.
In the Light Novel, Aqua defeated the weakened Beldia after Darkness cut through part of Beldia's armor. In the Anime, they had already established Darkness' extraordinary ability to miss stationary targets, so they changed the plot into that Beldia was severely weakened and could be defeated by Sacred Turn Undead.