Originally Posted by jekyl_hyde
Which makes me think Isshin took on some Arrancar, or possibly the trio of Tousen, Gin, and Aizen. With Toshiro's statement of how strong/powerful Isshin is (and being a Captain), to have Isshin lose to some ground fodder is utter ridiculousness... but we are talking about Kubo here.
Unlikely it's Gin/Tosen/Aizen. Isshin pointed out that Masaki killed a hollow that bested himself.