The reason why Oni is somewhat beatable is because he also has very little experience against top players, though in his case it's because he stopped caring about playing well probably because everyone around him totally sucks anyway.
Oni participated in the WC two years ago, so he must've picked up some experience against international players there. However, since he didn't play S1 and we don't know how early Japan was eliminated by France, it's hard to say how much exactly. Personally, I think the only one on the Japanese team whose experience is at least kind of comparable to Volk's (not equal though) is Byoudouin since he traveled so much - Duke is probably second in terms of experience. Oni has some, but apparently not to the extent that he could just walk over people who don't have any based on just that.
I'm pretty sure Tanegashima has at least some other technique or skill as the Japan No. 2 and going even with Oni. Sure Oni was not at full power but Tanegashima seems too cocky to be implying he is weaker than Oni.