What, so Yuu is suddenly less strong now and has to resort to cheap tricks to beat his opponent? Osada just doesn't seem like he should be that strong compared to Yuu.
You're talking like Osada is a weakling — even Izawa acknowledged that he's strong; he's a blackbelt in karate, competes in tournaments regularly, actually trains consistently at his dojo unlike Shougo, and also is more experienced than Shougo at karate
He's definitely much stronger than your untrained no-good street thugs
Nakamura mentions that, unless the sparring was set up to resemble a street fight, Shougo would usually lose to Osada in training
Osada is also a big dude, and has both great power and finely-honed technique
The biggest reason why Yuu is taking this sort of tricky underhanded approach to fighting Osada is — the difference between boxing (Yuu's main foundation in fighting) and full-contact karate
Boxing is all about bobbing and weaving, dodging attacks and throwing in counters
Full-contact karate, on the other hand, is about that short-distance intense blow-exchanging — karatekas compete by getting in close to one another and hitting each other with strong blows until one gives out; it's much more focused on blocking rather than dodging, and emphasizes the strengthening and hardening of one's limbs to withstand heavy punches and kicks
Yuu knows Osada will beat him out in a fair close-quarters exchange because of his karate and bigger size, so he has to rely on his agility and dodging to move around and make use of his environment like the poles and corners to limit Osada's mobility