I'm going to be a bit contrarian here. I don't think Harry is actually that good at magical combat. I'll bet that he's maybe top 50 in the council, which considering their casualties over the past ten years is not as great as it seems.
To use a combat sport analogy to explain, I'd say that Harry is a good puncher, but not a good boxer. Like a lot of very hard hitters, it's a crutch for him and he leans on it way too much. His duel with Hannah Asher was quite ironic, given that he was essentially fighting himself from only a few years prior.
We've seen several examples of far less -- let's use the word energetic, rather than powerful -- practitioners demonstrate that they could be more than a match for Harry.
It's actually a good thing for the narrative that he is so bad. Given the potential power he wields, if he were also very skilled, it would lower the stakes.
I agree. In fact, I'll give an example IN a combat sport.
A wise veteran fighter, as it happens, an ex-champ himself, faces an up-and-comer. Max Schmeling watches film, one of the first athletes to break down film, and when asked why, Max supposedly says "I think I see something."
His opponent is a young Joe Louis. Fast, strong, can hit. Joe Louis is Harry. Joe didn't have good management, was very rough on his skills, getting by on physical gifts.
They meet, and the supposedly over the hill Max knocks Joe out in the 12th.
2 years later, the rematch. Joe's team has been replaced, his new trainer is not an idiot. Joe's skills have been polished somewhat, he is not as crude. Joe knocks out Max in the first.
Max is a better trained wizard. Maybe even Carlos. not as much power, but uses what he has well. Well schooled, well trained. Skills gets him a narrow decision over raw power.
Harry is Joe. raw power carried him far, until he reached a level it was not enough. But Joe took a step back, and honed his craft. Got better teaching. With even somewhat close skills, his gifts would let him dominate a rematch.
Carlos might be close to Harry now in a fight, skills vs. strength, but with a little work, Harry crushes him.
(in case you care, early in his career Joe dropped his left after a jab - a common amateurish mistake. It invited the right cross as a counter. Max had a good one. After the loss to Schmeling they fixed this flaw. Louis then became one of the most dominant heavyweight champs.)