Enel was much stronger than any other vilian Luffy defeated so far. I personaly claim Enel was even stronger than Moria but that may be wrong. Luffy just had the luck of BEEING Enels weakness.
But there's my point again. Enel didn't crash and burn even though most of his abilities didn't work either. I'm not saying Crocodile was weak. Don't get that wrong. He definately wasn't. But he was weak for a shichibukai