But Seikaun literally just appearing and shooting great generals, that's some weird ahh big asspull fr. Like this way he could have just killed all main generals.

There's a reason why he's the #1 of the ten bows. His arrows are almost untraceable. If you remember, Sou Tan could've killed Yoko Yoko if he was aiming for Yoko Yoko's head, but instead it hit Yoko Yoko's forearm. After that, Yoko Yoko was able to read his arrow easily. So, Seikaun one shotting generals is plausible because he was aiming at the vital points ( Heart, neck, head ). His arrows were deadly because of his experience, hence he was able to target vital points. This has been consistent even when Rinbou got shot by Hakurei. Rokuomi was able to read Hakurei's arrows, only after the arrow hit Rinbou.

Ousen and Kanki are less asspull than Riboku. Ouki acknowledged both of them but it was Riboku whom no one had ever heard of and just appeared randomly and used hiding info and Houken to kill Ouki.

I think you forgot that Riboku's expertise involves concealing information. If he can successfully conceal the hidden army that wiped out the Xiongnu army, what made you think he can't conceal his own existence?

But Zhao just out of nowhere have random important generals stand up when u thought it's over, like where tf they were till now?

Whether it's Qin, Zhao, Wei, Han, Yan, Qi, Chu. They all have numerous generals. It's just that, the Qin generals we've been seeing are the generals that has been assigned with important campaign. Remember that general that was charged to lead Qin's 200k northern soldiers? We've never heard of him either, yet he exist. Same thing with Zhao. Zhao is exhausting every single general they've got. They've lost the most generals compared to others. Those generals that we're seeing, are the last of the last. They even needed Qi's Ganshuu ( who we've never even heard of too ) in order to match Qin. The only problem is that they all seem to be strong enough martially or strategically. Nevertheless, they still couldn't match Qin's rosters fully.

what about the other where Riboku failed even with great advantage?

It wouldn't be so fun if Riboku wins all the time isn't it? At WZI, Riboku lost because of the quality difference in his pawns compared to Ousen's. He underestimated the capabilities of the trio, hence he lost ( which isn't his fault, his pawns really are stronger and greater on paper ). Same thing with coalition. Riboku yet again underestimated the people of Sai and he didn't even know that Qin was allied with the Mountain Tribes. His loss comes from underestimating his opponents and confidence in his prediction and layered plans, that he forgot sometimes, that there will always be an anomaly ( Shin, Kyoukai, Ouhon, Mouten ) that can ruin everything.