The biggest issue is environmental. Animals today are generally smaller for many reasons, but a huge part of it is we have 1/3 of the oxygen as compared to the late Cretaceous.

A Rex could not sustain 6 tons of energy-hungry mass at 1/3 oxygen intake. Period. I’m not sure if it would die immediately, but it certainly couldn’t perform at its standard abilities.

A Trex could clearly take on dozens of grizzlies in peak performance, but I think in a “realistic” setting the T Rex would kill 3 bears, pass out from hypoxia, then get eaten.

If it was a rich oxygen ecosystem, then I’d say a 6-ton Rex clears at least 20 grizzlies. Maybe more.