Grizzly Bears are the weaklings of the species it would take more than 12 the T-Rex would just play dead anyway and the bear would walk away.
You need some of these bad mofo's they are the SAS of the species and could probably decimate a T-Rex one on one, the play dead trick wouldnt work either
Kodiak Bear, they usually are black which makes them extra bad but this one is brown for some reason.