Technically, the kitty should have made 2 separate RRs against the stun, presuming failure of either or both, he would not have passed out, but just been stunned (unable to attack, likely to run rather than just sitting there waiting to be smacked on some more), unless it is immune to stun (a common capability among demonic creatures).
And being stunned, while in the air, as a GM, I would
not have allowed it to complete it attack, BUT its momentum would have been considered a (different) bashing attack against that poor guy on the ground (although the arrows hitting it may have also shoved it off course, since it was in mid-air).
As for the GM lumping both RRs into one - He is the GM, so that is perfectly alright. The same for him deciding that the combined attacks knocked the creature unconscious.
As GM, it is up to him to adjust combat/critical results to match the situation, and he may have included other factors that he didn't tell you guys about (who knows it may have already been way down on hits and the two attack put it below zero knocking it out).
Thus, I have absolutely no issues with how he handled it -- afterall, it is his game.
