Machina as a species were vulnerable to electricity because they are machines, and electricity messes with circuitry. Wahl makes up for this by making it so that he can turn electricity into power for himself. Laxus got around that with red lightning either by:
1) Creating a type of lightning that is different enough from normal lightning that Wahl can no longer convert it into power.
2) Having a type of lightning so strong it overloads Wahl's system despite his defenses
3) A combination of the above.
It really doesn't matter what type of lightning is used, it cannot overload Wahl. Wahl is a machina so that is why electricity overloads his system. It is essentially the same as the system of a human body being overloaded by magical energy except we don't use electricity to fuel ourselves. Therefore,
Theory #2 doesn't work here. That would also make
Theory #3 invalid. Let me further explain what I mean. Wahl stated that he is immune to lightning because he can convert it into electrical energy for his benefit. Being immune to magic, thus being able to absorb it in order to power yourself sounds familiar, right? Slayer Magic. Wahl's magic is very similar to that of a Slayer. For example, dragon slayers can eat elements of their own and use it to power themselves. There is no limit. Saying that the lightning is so strong that it overloads Wahl's system doesn't work unless we can say that Natsu cannot contain an immensely powerful type of fire in his stomach. From what we've seen so far, Natsu has been able to eat all kinds of fire without exploding.
Theory #1 is possible but still not likely. Silver's
Ice Devil Slaying Magic allowed the element,
ice, to not affect him. This is pretty much the same as the element,
lightning, not affecting Wahl. Wahl's justification for immunity was because he could convert it. At the same time, Silver can convert his opponents' ice into his power when he eats it as well. So unless there is a type of ice existing that can affect Silver, I cannot agree with this statement. Nothing makes Wahl's and Laxus' situation special. In theory, they are similar. Therefore the other only explanation possible was that PoF and Nakama Power were involved.
Now, I will acknowledge Zancrow's type of fire did make Natsu unable to eat it. Because Zancrow's flames happened to affect Natsu, it meant that the type of magic
could negate immunity as it won't pass through Natsu's body (Natsu can't eat it). This always puzzled me because it contradicts what was explained since
Chapter 1 of Fairy Tail. Either way, I've always labeled this fight as nonsense too.
Either way, I wouldn't say that Laxus was weaker than Wahl. Laxus was actually keeping up with Wahl, and even managed to land a few hits against assault mode. He didn't succumb to his injuries either. He collapsed because his organs were failing, and he was in fighting shape after Wahl cleared the particles. Even if you say his victory was the result of an advantage, that doesn't change the fact he kept up with Wahl until the very end when he utilized that advantage.
First of all, I would like to clarify the effects of magical barrier particles. Do you think it consistently affects him like cancer would or would it be more like a stroke/heart attack where it happens periodically provided you don't die from it?
Secondly, I would like to differ on the part where Laxus has more MP than Wahl. By MP here, I will be using your definition which is what I usually refer to as magic energy. Anyways, assuming that Wahl does have higher MP than Laxus, then wouldn't consuming Laxus' lightning not affect or overload him as much? After all, having a higher capacity for magic would allow some room for more MP to be consumed.
It should also be noted that Wahl sustained no damage from Laxus throughout the entire fight as there were no visible injuries on him. Laxus, on the other hand, was injured. I agree that the magic barrier particles may have affected Laxus but he did get powered up from Wahl's miscalculation. In the end, his condition was worse.
What I'm interested in seeing is how someone can justify anyone else being stronger than Wahl if they argue that Laxus wasn't. Laxus has the 2nd best damage feat out of anyone on Fairy Tail's list, and hands down the best durability feats.
Lastly, just because Laxus isn't stronger than Wahl, that does not mean that we can't justify someone else being stronger. Laxus was at a clear disadvantage in this fight for a reason as Wahl is a direct counter to him. Laxus might be stronger than Natsu (I don't agree with this), for example, but Natsu would be able to defeat Wahl while Laxus can't. Reason? Just because Laxus is not meant for him. Like how Jacob is not meant for Lucy. The fact that Laxus used an unexplained spell, it showed that something does not add up. Similarly, Natsu could never defeat Totomaru. Why? Because he was countered in every direction. Who had to defeat Totomaru for Natsu? Gray and Elfman. Does that mean Natsu is weaker than Totomaru? Not necessarily...
I also can't see how we can say that Laxus has best durability feat.