I don't think Ursa's ability was broken imo but it was allowed to be exploited because the fireworks that stun the beast had zero cool down and no limit on how many you could purchase (as long as you had enough ingnots)
Without being able to perma stun the beast, Ursa's stacks would reset because he would either get stunned by the beast or he would have to kite some of the magic abilities from time to time.
Without being able to perma stun the beast, Ursa's stacks would reset because he would either get stunned by the beast or he would have to kite some of the magic abilities from time to time.