Bonkle thoughts, not so late edition:
Mata Nui Matoran, especially Ta-Matoran, are absurdly skilled fighters for a random generic Matoran.
Why? Vakama. Vakama had an odd assortment of mentors.
First, obviously, Toa Lhikan. Not for too long, but they were on talking terms even when Vakama was a Matoran, so he might have gotten in a few tips in even then. Maybe he was even Lhikan's designated maskmaker and in extension probably to all the Toa Mangai then. That's a lot of adventures he could have heard about at the very least.
His teammates obviously. We see at least one instance in the book where they spar.
Sidorak. Compared to a Dark Hunter, he is a shit fighter, but still a moderately good fighter as I recall it from the books, or at least had the balls to lead his Visorak in the person. Gladiatoral fights are also a national sport to steltians, and he was friendly or at least polite with Vakama. There is no way he didn't have holos/videos on him to watch in his downtime. Obviously his Commander of the Visorak Hordes, who is a fresh Toa, must see the best ones. For educational purposes of course. (Roodaka appears mid-session to point out mistakes and incorrect forms, and gets the most upset at the referee when they make a shit decision. This is the only time Roodaka shows any genuine interest in Sidorak. Roodaka is very disturbed by this afterwards.)
Roodaka. She is a skilled double spy (see, she scales a DH bastion's wall like no-one's business). She has both Brotherhood and Dark Hunter training. She captured four martially highly skilled Toa Hagah and made such an impression on Bogaraks that they'd rather jump off a tower, than displease her. Roodaka has high standards even with her casual hipocrisy, so it's safe to assume she at least taught Vakama how to dodge properly so that he would be an asset for longer.
The whole team of previously mentioned Toa Hagah. After the Visorak Hordes are gone and while they are building the airships, I'm sure the Rahaga made a point to prepare their Toa Metru siblings as much as possible (wich probably included both Toa training and any random Brotherhood related skill or knowledge they deemed useful).
In conclusion, Vakama has the rarest hybrid dark-hunter-brotherhood-classic-Toa training in the whole MU. Which he imparted on his Matoran. No wonder that the Inika/Mahri kicked were as good handling bad guys as they were.