Just watched it, I agree with you, phase 2 needs a little more frame, mainly on the last part of the animation where undyne goes down before the animation starts again.
The animation of phase 3 is smoother than the one in phase 2, but I think it should also have another frame at the end of the animation to make the animation of the arm with the spear and the torso smoother.
Other than that, good job on those remixes! I really liked them!