Well Shazam could be as strong as Superman. He hasnt reached a limit yet. Hes stronger than WW. But now hes, a bit lower than Supes. Kingdom Come was a good battle between the two. And Superman did pretty well against the lightning. He just kept getting up. Talk about persistant.
Just because magic is involved doesn't mean Superman will lose easily. Superman would win since that magic will not knock him out easily.
"In Adventures of Superman #581, Superman allowed Adversary, a powerful magical foe, to hit him as hard as he could, without retaliation. Superman was literally punched halfway across the planet- from Metropolis, he literally landed in China- without any injury or loss of consciousness "
"Action Comics # 762: Superman harmlessly shrugged off a blow from the magical demon Etrigan. The blow actually knocked Superman from Earth's surface all the way to the moon (239,000 miles), a distance that is about 10 times the diameter of the Earth. Superman was virtually unfazed. In this issue he also shrugged off magical demon-fire (which he did previously in Action #589), and his super-lungs harmlessly inhaled the demon's magical gases. "
Kingdom Come #there was only one: A more powerful Superman, from soaking up the sun's rays for a very long lifetime (i.e. He was practically invulnerable to Kryptonite) was laid low by three thunder bolts from Billy (not dead, but severely harmed).
Marvel was created to be practically Superman's equal...minus the magic, Supes would win but it would be a good fight. With it, Marvel has the advantage.
If Superman shrugged off powerful magic, it was bad writing. He's vulnerable to magic. Period. If something contradicts that, then DC should change his vulnerabilities. So far, they haven't.