The Toa Nuva are the most powerful Toa in the Matoran Universe, perhaps only rivaled by the Toa Inika. It is odd, then, that the Toa Nuva lost fights against Rahkshi and, most famously, the Piraka- especially considering they successfully combated more powerful enemies in the Makuta of Karda Nui later on. Further, the Toa Nuva (and other, weaker Toa) demolish countless Skakdi and Rahkshi throughout both Teridax’s reign and the Battle of Bara Magna- the same enemies they had fought one-one-one before and lost to.
I’m going to attempt to justify these inconsistencies with primarily speculation as well as some canon material.
The Piraka are physical powerhouses who can tolerate most physical and elemental attacks: this is a Skakdi’s nature.The Piraka aren’t normal Skakdi, though. They’ve received Dark Hunter training for over 5000 years, and some, including Vezok, Reidak, and Avak, are sent out to complete gravely significant missions, such as releasing the Kanohi Dragon and bargaining with Turaga Dume, or in Thok’s case, kidnapping him. The Skakdi are sent out on these important missions by The Shadowed One despite betraying him, and thus it can be concluded that choosing them for the job was probably based upon skill and merit.
Even before joining the Dark Hunters, The Piraka must have been significant in some way or another. The Dark Hunters have a general policy regarding Skakdi: they’re too treacherous to be trusted. After Spiriah’s experiments, Makuta Krika took over in his stead and quarantined the island. Skakdi were not allowed to leave; somehow the Piraka were successful in either leaving or causing large enough a disturbance as to be spirited off Zakaz by Dark Hunters.
(Quotes from BS01)
“He engaged in an argument with one of the order enforcers, which escalated into a series of disputes that erupted into a war on the island, resulting in much of it being destroyed, and all the inhabitants except himself fleeing it.”
“Almost immediately, he was a problem. Complaining, questioning authority, scheming against his betters… all of this earned him his share of dangerous missions and a number of sessions with the masters of pain. Somehow, he survived all of it…”
-The Shadowed One
The Piraka are far more combat effective than regular Skakdi, and thus are not comparable to, for example, the horde of Skakdi defeated when “[Brutaka] sent [a] structure toppling down on a mob of Skakdi. When the dust cleared, all of them were trapped beneath the rubble.”
Can you picture the Piraka being defeated all at once by a toppling structure?
(Well, maybe Hakann(see the comics))
There is a reason the Piraka are so infamous, even amongst their own people. Their predisposition to excellence, in addition to extensive training by dark hunters, something no other Skakdi received, is why the Piraka were able to defeat the most powerful Toa Team in existence, when dozens of other Skakdi fall before them.
-The Rahkshi of Mata Nui-
A battle that should have been won with ease, but was instead hard-fought, was the fight between the Toa Nuva and six Rahkshi upon Mata Nui.
First and foremost, it should be noted that Teridax intended only to capture the Mask of Light, not to harm the Matoran or Toa nor damage the island. This is the first and only serious move against the residents of the island. He has been recorded as reasoning that he hadn’t sent the Rahkshi to Mata Nui due to their destructive nature, and only the advent of the discovery of the Mask of Light caused him to reveal his hand. This is probably true: the Rahkshi, unmonitored, would have probably ravaged the island and killed the Matoran and Turaga(In the Toa’s absence). Teridax intended for the Matoran to return to Metru Nui safetly. If Teridax were controlling and directing them with greater focus and precision, he could mitigate their destructiveness, and- though this is highly speculative- increase their combat effectiveness.
If Teridax has more mental focus put upon a few particular Rahkshi, it would seem likely to me that his heightened influence would strengthen the Rahkshi, particularily if he is influencing their specific actions.
The Rahkshi army Teridax unleashes upon Voporak have one objective, and a simple one: engage Voporak. They don’t need much direction; they don’t need to be monitored. Therefore, they’re probably going to be less effective than a Rahkshi Teridax is keeping his focus on. These six are bolstered by his presence, and thus are more able to stand up to the Toa Nuva.
Sorry this whole thing deflated at the end, I’ll probably edit it in the morning but I’m tired and it’s three in the morning.