Is it another bug?
Yes, but it's not relevant, as I said – let it be. We need this hero in one piece.
And how else can we complete this battle, if you don't mind sharing?
In general, it involves planning a route where we drag two stacks of pikemen along for a while, after which one of the stacks needs to distract another of our gnolls. There's an element of chance involved (choosing between two targets that can be reached in the same number of turns).