...all i can say is you are clearly overestimating our current powerlevel, and severly underestimate Diana's powerlevel.
You say we are far in the plot but we are only in season 1 out of 3 seasons. I have stated before we have a whole lot of growth and power-ups to go through, we may be a powerful being and have a vast potential but currently we are far from the most powerful in existence, theres lots of powerful creatures that would squash us like an insect currently.
Yes we have been trained, guess what? So has Diana, we have been trained on and off for years on the road, Diana has had access to the best combat and martial artist experts the nobles can afford. she has more combat experience than us by quite a bit.
I will admit if we have not been holding back and just fighting for survival from the get go, using everything at our disposal, then we could likely have defeated her.
But then again the side effect of her water-infused shockwave attacks took us by surprise and disabled onyx and onyx is a big part of our current power gains.
Also you say we run out of energy fast? Eh I'm pretty sure we just ran for hours and hours even saying in-game we are depleted somewhat when we arrive, using superspeed is not something to be taken lightly and takes a severe toll on the person the only reason we can sustain it is due to our already vast energy reserves.
I personally think the fight with Diana is great the way it is and portrays both her strengths and our weaknesses, if you have made up in your mind that we are all-powerful and cannot be defeated, that is your own expectation and certainly not the case.
It also highlights the youth of our mc, being only 20 years old and powerful beyond his peers, means he is prone to arrogance and overconfidence which when faced with a rare and powerful opponent makes for an unfortunate outcome.
I'm not sure if that answers your questions or not but I think I've covered the scene pretty extensively by now