Strange beating Mephisto is kind of an odd things... Mephisto once told the Surfer that he never uses his full power against mortals, because if he destroyed them he could not take their souls.
I've been thinking about the whole days of vengeance storyline, and I've kind of come up with a theory about it, concerning the Spectre.
The Spectre, as the embodiment of God's wrath, draws his power directly from the Presence. However, he gains more power when he is doing something that god ordered him to do... ie, he can draw upon enough power to accomplish whatever is necessary. however, when operating on his own (as he was during days of vengeance, because God had not ordered him to do it) he was at a much weaker state than he is in many of his other showings... at only his base power levels, using only the power he had within himself, by virtue of who he is. this theory may be completely off base, but it would explain why nabu was able to survive his attacks so many times. agree or disagree?