Something has just come to mind. Here is the text on the Soul Eater card:
[This card can only be played by “Riku.”
If this card participates in a victorious challenge, your opponent loses one additional HP.
Note: This effect cannot come into play more than once per turn.]
The part that gets me is the last line. Does it only apply to the effect of multiple Soul Eaters used? Or can you not use Soul Eater's effect in addition to the effect of Way to the Dawn? (I figured this was on topic, even though it probably belongs in the Rulings Forum.)