Eh i won't compare him to Wahl to be honest,let me point out my arguments
1)First of all Machias Race is inherently weak to lightning
Though Wahl had resistance to it,but when Laxus bypassed it with his Red Lightning he obviously fell for the natural weakness of his Race.
2)God Serena was considered to be a very strong fighter in his prime by Gildarts himself
3)FL takes time to charge,if God Serena just stands idiotically like Mashima's villains while Laxus charges FL that's a different question but if he has even a bit of intelligence he will attack him immediately without giving him a chance to react.
4)Laxus is weak against Fire Magic/Fire DS Magic.
So if God Serena uses Purgatory DS Magic then Laxus will be in deep trouble.
5)If Laxus can't use his Lightning DS Magic,doesn't get time to use FL Law and is weak against Fire Magic what choice does he have?Only physical fight.But God Serena will spam him with flames and other magic if he tries to do that,