Polar Bear vs. Megistotherium osteothlastes

Well, Wikipedia cited "A biomechanical constraint on body mass in terrestrial mammalian carnivores" (link) to claim a figure of 500kg.

I at least know the hyaenodontid would give a bear hell at alike weights.

Hyaenodon horridus had a bite force of ~2,650N at the canines at what people believe to be at a mass of ~60kg (though blaze from Carnivora believes H. horridus was probably no more than ~40kg). If its fellow hyaenodontid Megistotherium was similar, at ~500kg it would bite with a force of ~10,892N (assuming the ~2.65kN figure would still only apply at ~60kg and thus at 40kg the bite force would be less than the aforementioned figire) or ~14,273N (assuming the bite force figure would then apply at ~40kg for those specimens, thus giving Hyaenodon a bite force that was even more powerful proportionately). Hyaenodontids also had proportionately large heads for their size and Hyaenodon was stated to have canines ideal for slashing (it's likely Megistotherium did too), so they've got some deadly weapons.

Megistotherium would then at the least be a very tough opponent to a polar bear if they were really of similar mass.