It's simple math [MAY CONTAIN SPOILERS ABOUT NIDHOGG]:

Warrior of Light VS Nidhogg + Eye: We win (the VS works like a = in this weird math case)
Warrior of Light VS Nidhogg + (Eye x 2): He would have double the power (since he's "half of himself" without one of his eyes)

Warrior of Light + Eye VS Nidhogg + (Eye x 2): This is the tricky part, you have to put one of his eyes to make the math look like the first example:
Resolution:
(Warrior of Light + Eye)/Eye VS Nidhogg + Eye: Eye/Eye = 0, so the Eye we had countered his extra Eye.

In the end:
Warrior of Light VS Nidhogg + Eye: We win.

And THAT is how he lost. My math is totally logical. No contest.