The AMD's trap ("Dust")

This scene isn't a battle, but it played a major role during the events of Season 2's beginning, and, counting Asriel's few attacks against Bete, it can be considered a short battle/conflict. This event includes the characters Sans, Asriel, Bete Noire, and Jessica Grey.

Context
After the HSO meeting where the incident with Asgore happened, Jessica Grey, the AMD director, was blinded with the wish of revenge. This allowed Bete and Akumu to seize the moment and ally with Jessica Grey, as their purpose is to make sure that monsters and humans would never live in peace.

Furthermore, when this trap occurred, Asriel and Sans were looking for some proof of Papyrus had seen something, and not just dropped what a crane was carrying unto a crowd of unsuspecting civilians for no reason. So, they decided to head to AMD's security camera room to find some answers.

Battle
After Sans lashed out at Frisk about his empty promise, he decided to go to the AMD's security camera room with Asriel.

They discovered a strangely secluded folder in a camera's files that show Bete doing strange things, and appearing in many unusual occurrences; Such as teleporting outside whilst she was supposedly sleeping, or appearing in strange places whilst others did seemingly random things for no reason whatsoever.

This then led them to believe that Bete was the one behind everything.

Sans, upon realizing this tells Asriel they need to warn the others, but before they could leave Bete appears, who gives some villainous monologue and attacks the duo. Sans and Asriel then try to flee, but - Jessica having already hidden a dark corner of the room, shoots Sans with the AMR, temporarily rendering him unable to use his magic. Bete seizes the opportunity in place and then tries to take Asriel's soul by manipulating him to see a hallucination of Chara whilst she throws her spear at him.

Luckily for Asriel, Sans was able to just about generate enough magic at the moment to teleport in front of him and take the hit. But Bete having gained a soul anyways, feeds Sans' soul to Akumu, causing Sans to turn into dust. During this very moment, Asriel snaps out of the hallucination, and realizes what happened, infuriating him as he prepares himself to attack Bete, only to remember what Sans told him about warning the others. So instead of attacking with this charged up energy, he blows a hole through the wall and leaves, sending debris flying all over the place and covering his tracks.

After this stark interaction, Bete tells Jessica to give her the HATE Vial to which Jessica refused. But Bete, having given her an order rather than asking, inquired her to give the hate vial again in a much crueler manner, before resorting to torturing her and causing her to relive her memories of Alina's disappearance.