My money's on Lisowicia because this. https://www.youtube.com/watch?v=RlK68pX85Qo

Lystrosaurus vs Oviraptor -Full Scene (Lystrosaurus Cuts Off Oviraptor's Head!)YouTube

I mean, I doubt it would be able to cleave off Diprotodon's head but at least its bite would cause fatal wounds that it would kill it.