"i can't just give up now...." "i've come too far to stop...." "sorry undyne, but i gotta do this..." "no hard feelings pal..."
This is the waterfall encounter. he manages to kill undyne with a sneak attack and gains a huge jump in power from this. he does hate how he had to do this but he can't stop now, not when he's this far. now, he doesn't have any spear moves from undyne, but he does mimic them with his bones. (also, once again, got no clue who to credit for the music)