For me, it's not about durability. So, nevermind if the opponent is physically strong or physically weak. It's about Magic Level.I can certainly see FL one shotting half the Spriggans (those with poor durability, I don't see Ajeel in this category btw) in a 1 vs 1
For me, the definition of Hax spell is non physical effect attack. It means that it can't be counter by physical defense. Many of the Spriggans are Hax wizards despite physical wizards such as Fairy Tail's members. Their physicals are weaker and this is why many of people think they are weak. But you have to take account that they are despicted as having tremendeous MP. Hax Wizards are the opposite of physical wizards. They are different but certainly not weaker. They are weaker, only if the physical wizard has better MP than Hax wizard.
If I can put analogy, I can take Marvel Universe. Hax people are such Psychics. Physical people are the like of Juggernaut, Superman's like with tremendeous durability, such to tank atomic or nuclear blast.
Professor Xavier, one the greatest Psychic Marvel Verse is physically weaker than normal human. But his powers can only be counter by another Psychics around his level or people with very strong will. You can make a Superman guy like, a god power, if his mind is weak, Xavier can shut him down in second. Juggernaut is near indestructible but without his helmet, he can't face Xavier, while he can physically take blows from the like of Hulk and Thor.
The power of will is represent in Fairy Tail by the level of Ethernano. Spriggans have the best level here.
Fairy Law is a hax spell like Xavier's psychic vaccum. He can't affect Psychics stronger than him (Phoenix) or people with stronger will than him (Hulk's berserker). Makarov is in same way with the Spriggans.
The most representative in Fairy Tail, is Natsu vs Dimaria. The rage grew his MP level to the point he could walk under Dimaria's hax. Same as Brandish hax couldn't affect amp Nineheart where he could be physically blast by Natsu's strength.
Hax and Physicals wizards are opposite but both have limits : 1 by MP level, 1 by durability and stamina.