Elusiveness says "When defending, you may receive 1 stress token to choose 1 attack die. The attacker must reroll that die. If you have at least 1 stress token, you cannot use this ability."

If my opponent has already spent a target lock to reroll all attack dice, can I still use Elusiveness to force him to reroll one? The side bar on page 12 states pretty clearly that "a die that has already been rerolled cannot be rerolled again during this attack."

I would still allow it, as the card uses the phrase, "The attacker must reroll that die," [emphasis added].

Two things wrong with your interpretation: One, cannot always wins. Two, read the modifying attack rolls section more carefully - the defender has to do any and all of their modifications of attack before the attacker does any of their effects.

dbmeboy said:

That's right, the Elusiveness reroll would happen first, and once it is rerolled, it cannot be rerolled again by anyone.

Thanks guys. My brother said the same thing when I called him today. Sorry to spam the forums. I seriously looked for this topic for 45 minutes before finally deciding to post the question.

Now worries. If you are in doubt, ask away.
And by creating this thread, I'm pretty sure that someone else might have gotten a clarification on this matter.