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when you select the attack option, if he is at 1 hit until he dies, it will be yellow (this is also indicated by him collapsing on to his knees). when this happens you have 2 choices: either attack him once more and kill him, or click the spare button. all monsters and bosses in any undertale game, the original or a fan made, are like this. (in most undertale fan games and in the original you can also just spam the spare button enough times or use the right act buttons. but in this fan game in  particular it doesn't let you do that.)