Really? How does Weakness Magic have anything to do with this situation? Wahl has two types of magic. Nullification Magic and Weakness Magic. Weakness Magic had nothing to do with Laxus except the immunity to lightning. But that is a different topic. The nullification aspect is what I'm talking about here. Like you said, there are only two conditions in which you could dispel Rune Magic. The first being that you satisfy its rules. The second being that you are able to create a new one. None of which applies to the canceling of Magic Barrier Particles.
To be honest, I know exactly where you're coming from, but it's easy to misconstrue the context here.
Weakness Magic is
everything in this fight, my friend. Contrary to what may seem, Wahl doesn't have nullification magic per se. If the nullification magic you're pertaining to is Wahl nullifying the MBPs, that's not nullifying -
that's a side effect of exploiting the weakness of that specific Rune Magic. Since Laxus set the rules to dispel that last rune barrier - the condition being to remove all MBPs - Wahl certainly did just that by fulfilling the condition Laxus set.
That outcome actually stemmed from Wahl's weakness magic. The Weakness Magic is a veritable counter to everything - Juvia's water body? Voila, Wahlbot has burning steam. Gray's ice? Voila, Wahlbot suddenly has fire. Elfman's sluggish powerpunches? Voila, Wahlbot suddenly has super speed. Those are no different from - Laxus has rune barriers? Voila, Wahl suddenly has MBP removing abilities to fulfill the condition in order to remove that barrier.
The Weakness Magic endows Wahl a diverse deus-ex-machina skill set to pinpoint an enemy's weakness and is different from battle to battle.
That's called exploiting every magic's weakness, and the Rune Magic was no exception. Wahl just exploited its major weakness - just fulfill the condition and it will disappear.
If his magic is indeed nullification, he could've just rendered everyone's magic ineffective and could've just proceeded to lolstomp everyone. He could've turned Juvia's body to ice, he could've reduced Gray's ice to a puddle like Sugarboy from Edolas did, hell, he could've disabled Elfman's and Lisanna's TakeOvers to prove a point.
But he cannot, since his magic is Weakness, ergo - countering attacks with the thing that they're mostly vulnerable to, which will render them ineffective.
The only true nullification feat we've seen in FT so far is Erza and Brandish being in those anti-magic handcuffs (unless I'm forgetting something?). That's nullification. If there's any panel or feat of Wahl "nullifying" or dispersing something other than the MBPs, then I'll have to concede and admit that I misunderstood the context, but Mashima's panel placement in those scenes is pretty deliberate.
You're missing the point. I'm not arguing about whether the condition to nullify the Magical Barrier Particles satisfied the rules or not. I'm simply asking how was he able to cancel out two different types of magic with totally opposite effects. The fact that both coincidentally had similar names should not be a factor when they aren't even remotely the same in context.
I'm sorry, but you're the one who's missing the point. I'm assuming that you're referring to this post?
To start off, Magical Barrier Particles (MBP) which is what caused Laxus' illness is Anti-Magic or Anti-Ethernano Particles. It is a form of poison to mages since they use magic. Originally, these particles were associated with Curses which is opposite to Magic.
On the contrary, the Jutsu Shiki (Letter Magic) which is supposedly a form of Caster Magic that uses magical runes to create barriers which most people refer to as Rune Magic. Note that this is a form of magic and that it literally creates a barrier.
So the confusion between both types of Magical Barrier Particles lies in the fact that one is actually a magic barrier (Anti-Ethernano) that takes the fotrm of particles (Mist) while the other is pretty much just magic particles (Runes) that make up a barrier (Shield/Boundary). So same name, but totally different in context.
Now now, we're going to silly semantics here. Wahl did not dispel the Rune Barrier per se. He just removed the MBPs (an ability granted by his Weakness Magic), which caused the Rune Barrier to disappear. Since he, you know - fulfilled the condition Laxus set. Once came after the other; they were not simultaneous.
The question you should be asking was why Wahl was all gungho and stupidly desperate enough to get out of that Rune Barrier.
Bottomline - Laxus outsmarted Wahl on all accounts and won on impressive terms. Laxus knew about Wahl's weakness magic and waited for the right time to use that ability for his own benefit.