This game... See, there's a boss battle in chapter 3 even if you're running pacifist that is practically impossible even on easy mode... I've played deltarune and undertale through and that fight... it's infuriating. I've been trying to beat it since that update was released, additionally the wiki is no help as it says you can beat him just by talking. I never played genocide through in Undertale or beat any optional bosses in Deltarune so this fight was just tedious for me to even attempt. I cannot recommend this to players who want to be able to play it all the way through and aren't the type to seek high challenges.
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.