Nice plothole there Miura-sensei:
Even though Kogarashi and Yuuna know beforehand that Ouga was coming it just doesnt make sense that Koga didnt use his limitless training to make himself stronger and the recover his long lost spiritual power with the replenish stone or the spiritual crystal, then he just needed to be possessed by Yuuna and btfo master-san.When he fought Byakuei Ouga stated that Byakuei SP was at least 300 million (Ouga's max was 120 million) so if Kogarashi without training and just using the replenish stone and Yuuna's possession was able to exchange some blows with Byakuei, so there's no way he could lost so bad in a battle with Ouga.Story is not the point of Yuragi but dang Miura at least try make it a little bit better.