Has Shiva ever been depicted as fighting mounted on Nandi in his bull form?
The Banasura fight as related in Harivamsa is one example. Are there any others?
This chapter of the Karna Parva of the Mahabharata describes Shiva first riding a chariot pulled by his bull, and then riding the bull itself, when he goes to destroy Tripura:
After, O monarch, that best of cars had thus been constructed by the gods, O tiger among men, for grinding their foes, Sankara placed upon it his own celestial weapons. Making the sky its flagstaff, he placed upon it his bovine bull.... Indeed, when Bhava, riding on the car, set out towards the triple city, his bull uttered tremendous roars, filling all the points of the compass.... During the time the car had sunk and the foe had began to roar, the illustrious Deity, endued with great might began, from rage, to utter loud shouts, standing, O giver of honours, on the head of his bull and the back of his steeds.
And by the way, there's some indication that Shiva's bull is not the same as Nandi, as I discuss in my answer here.