Well, if you you go buy the name of the next episode, the girl who beat everyone in the last wave, and the new magic, you could probably guess they are on the other team. You could also guess it'll be about saving both worlds.
Sorry, I'm just guessing. But I don't think the other side are bad guys. Did you see the translation scene both L'arc and naofumi used the same translation ability. This isn't a cliche anime so both teams could be "good". Think about it this way. If the waves happened on earth and another planet...
And the only way to stop the waves on earth was to make sure planet b lost, you'd fight against their heroes. You don't hate planet b, but earth is your home. You'd probably pray there was a way both planets could win, but no one's found it. This is naofumi's story, and I bet he will.
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With the introduction of Glass, I'm beginning to think that there's some kind of "BBEG" (big bad evil guy) running an army of sorts. Glass seemed, to me, to be some sort of "Captain" or "General", sent to evaluate Naofumi, because he's both the strongest of the four, and the one whose made the most progress since the heroes arrived.
We also can't forget about the previous Shield Hero- apparently he did something which, either directly or indirectly, caused Trash to experience hardship ("I'll lose my family, again" etc), and may have also been noticed by the BBEG in the last set of Waves. Which has to make you wonder, how often is the world threatened that there are 4 specific magic weapons kept on-hand for four specific heroes? Looking at Fitoria's failing memory, the assaults probably occur no less than 100 years apart.