Y'all keep complaining that he isn't showing off enough, but it's not like he was anyone that special in the future he came from. He had no talent, and managed to become a passable soldier because of the advances in magic that he brought back with him, but there's a couple of things holding him back right now. First, still untalented. He'll have to work several times harder than everyone around him just to see the same benefits of the things he helped "pioneer" this time around. Next up, he's still a child. The people we keep seeing him train against are not only extremely talented, such as his father or this chick he just fought, they're actually adults. An adult and a kid with the same skill level, the adult will have the advantage. These adults also have the skill level advantage, so, he's not going to be showing them up in any sort of spectacular fashion just yet. Another thing, his greatest asset right now even without that soul eating chick's aid is his ability to produce a scary level of dark magic by using her as the image for "darkness." But he's been told not to use that on other humans by his teacher. Frankly he has more hands tied behind his back than humans are supposed to even have at all, so it's not like it's weird that he's getting rolled by some of the people teaching him.
TL;DR: He wasn't someone that could easily defeat these people before he got timey wimey'd, he's not gonna suddenly be a child prodigy that can defeat them before he's finished training. He's above average perhaps because of his knowledge from the previous timeline, but the truly talented will still outstrip him if they're not being lazy. Maybe after he lets that thing eat the dragon soul we'll start to see some real results. Her helping him was the only special thing about him in the first place.