I don’t want to say impossible, as it probably could be done, but you’d need at least a platoon of bears to swarm him of all differenty types.
I’m not convinced that the bears could do that much damage to the T-Rex’s hide, or get close enough to do it, as he’s just going to steam them at the start and knock them flying, which will put a load out of action for a start.
I suppose the bears could counter this by having a loose understanding of Rugby Union, with Grizzlies propping and Polar bears in the second row, they could be able to take the impact. But even then, the T-Rex is the size of a bus and can probably go as fast as the average number 9, so that’s a lot of blunt force trauma and the whole line of props and second row are probably going to have their necks broken.
Still, a rolling maul might force the T-Rex back, so some smaller and quicker, but still lairy black bears could flank him and distract him, allowing a Grizzly shcok bears to start giving his canister a good shoeing. The problem is, the T-Rex skull is so massive, I’m not sure if they’d be able to do enough damage before he turned round and started biting the **** out of them.
Surrounding him might be a good idea, but you have to remember that any flanking manoeuvre is fraught with difficulties of his tail giving them a cheeky swish that would do some serious damage itself.
Maybe a pack of ninja style pandas could use their skills to sneak up on him while he’s battling the big bears and take out his eyes. That would be a big disadvantage. But then again, his sense of smell is pretty good, so he might just start head butting the bears and wind milling his tail about.
Tough job on for the bears if you ask me.