Not by having a straight-up fight like the armored dinosaurs, but my idea is that when a theropod charges in for an attack, the hadrosaur can use its tail as a defensive measure, attempting to hit the theropod with it as it spins around to flee or swinging at the theropod as it changes direction while being pursued. Have it be a random behavior to give it more naturality. Against small theropods the hadrosaurs could do that as well as try to simply charge past and/or trample the attackers, not killing them but dealing serious injuries that require a trip to the vet building.