Master Chief beats your average Guardian despite the fact they are technically superior, because he has "luck". A guardian with a large enough "legend" may stand toe to toe, or at least come close. If he could hypothetically fight the mysterious stranger, their luck would at best cancel each others out (continuity only demands one hero from the "royal" "lineage" inherit the mantle and face Dark) and he would lose. Chances are the roots of the timeless hero are much deeper now though, and she would be luckier than him.