On try 15, I actually managed to beat it(on normal, obviously). The game is extremely well-made, the boss fight is not different from those in Undertale. The game itself on normal isn't that hard. The hardest attack - definitely on the last stage, the one where Sans is throwing bones and Garfield is shooting at you(still have no idea how to dodge it). In general, the game is awesome, and, unlike many other Undertale fangames that include sans fight, beatable.
P.s. Would love to see 1 more stage where Sansfield goes beast mode and the attacks become even harder.