Terranort actually kicked everyone's ass at the KG in the game and I feel he was the strongest darkness in the group given Aqua/Ven/Sora together had trouble defeating him.
I mean.. not
really. Aqua (who already fought this guy and had him on the ropes)
actively did nothing but gasp, stare, and promptly disappear. Riku and Mickey didn't even bother to move, or do...
anything but scream. Lea and Kairi were pretty useless in general during that section of the game. Lingering Will (who's only about as strong as Terra himself) always manages to give him a beat down. Sora's just inconsistent, and
also barely did anything. I mean, he
in the cutscenes got knocked down
hard by Siax, and kicked to ground by Xemnas (two people he's beaten before), just as easily as Terranort knocked him away. All things considered, I just don't see him being
that much of a threat. That whole scene just felt like a textbook case of Cutscene Incompetence, and a very stiff one at that. I said this a couple of pages back, but Ventus litterally ran up to Terra as if he wasn't
just told by Riku that Terra was with Organization. Nothing in that scene really makes sense tbh.
I'm also not sure we can say how much trouble he himself gave them in his tag team fight with Vanitas since it's a different dynamic, and story and gameplay don't always translate in this series it seems.
That said, I do agree it would've been more interesting to see Xehanort using Terra's body instead of his own for the finale (though maybe that would've complicated things too much?).