Do you mean sans? If so, i was in the same boat. I had only ever done undertale and deltarune pacifist routes. Its literally all about retrying and retrying. It becomes less of a challenge if you can get the timings reight, its just patterns. Theres no secret attack that can catch you out, its the same attacks over and over. Using the boost from pressing z really really helped though. Some attacks you need to spam it to win.