@Solipsist Except for the fact that demons are way stronger than humans and he only got hit with one spell that he underestimated. He failed to finish off the demon general because his holy magic is the only thing he had that would be super effective and he only has one "holy" spell, and he used up all the magic he had left after killing his entire army to cast it. It's possible that if he had more mp at that time he may have killed Rajas as he had to hold back some of the power of it. He was also trying to capture his target not kill him/her so he didn't go all out. After all if the culprit dies and the people don't come out of their comas that would still, technically, be a bad thing even if the comatose patients are not particularly liked by the public. Not to mention that there are other people involved, like the one that ran away, so capturing and interrogating the person he was fighting would be the best way to get all of them, besides setting a trap and getting lucky enough that you capture all the culprits.
@Knighto His magic works fine, it just isn't that effective on demons due to the fact that they have protection from their "god", which means only spells powered up by the goddess are able to pierce through with full strength so he has to use much stronger spells to kill them than he thought he'd need to. He's also not familiar with all the magic in this world so he wouldn't know what every spell does, and as far as we've seen he has no experience with dark magic from this world so he didn't have a way to properly estimate the power of the spells, except maybe comparing them to some from Earth, which might not even exist. Oh, and lets not forget that he said he isn't all that strong for a magician from Earth, and that his dad would've literally been able to one punch Rajas, while it took most of what he had to put him on the verge of death.