He should be significantly stronger than he was in his past life and he is effectively at least 40 years old in terms of experience, in addition to his experience fighting demons on the front lines. Yet, he is far from beating the knight captain and that black mage? It seems quite a bit off. Everything about him is better than it was in his past life. How the white flame method works into this is a bit vague and he was shown to have cores in his past life, which seems like a mistake since it was stated that the cores are specific to lionhearts. You could say that he's weaker because he's training the white flame instead of what he was using before, but in the first place it's superior and he combined it with his own technique. The cores he has now in addition to everything else should mean that he's at least as strong as his past self.
This isn't really a massive deal, but it takes away from the significance of his past life which has been a huge point in the narrative. I just wish there was some vague explanation given, but I don't think there was any.