When you take the side of Chara, there is only one hero to save the remnants of the world. Sans fight is considered impossible even compared to Photoshop Flowey. A bunch of hours or days are going to pass until you figure out how to dodge.
The skeleton uses Gaster’s blasters and bone attacks similar to Papyrus, but lots more violently. He will haphazardly change your soul mode from red to blue and cast a hail over your head. You can’t play nice around him: asking for mercy doesn’t provoke any shiver of pity in his heart. Instead, he will make your buttons a part of the bullet board and will constantly get you amid the turns.
To end this game, you must be really determined. When the skeleton plays you like a doll, and your attacks seem to cause zero damage, only a strong will pushes you to the edge.