Traveller characters don't hold up well against getting hit by things powerful enough to penetrate their armour. Combat efficiency goes downhill pretty quick when you start taking damage. A typical D&D boss fight where everyone stands around trading damage tends not to work out so well, unless the dice fall exactly right. Somebody might get hurt.
With my group, you'd really have to force the encounter if I wanted it to play out as a melee fight, as they'd be thinking of ways to not have to fight the bear, and still get what they want. Or to one-shot it with a giant gun. Which is fine, though sometimes they mess up and end up in a pit with a tentacle monster or covered with giant jumping spiders. I'd play it as a cat and mouse game instead, with some NPCs along to be the mice and show how the monster works, forcing the players to out-think the bear.
As a player, I would try to lure the bear out with bait and shoot it from a distance. It has to eat, therefore it has to hunt, and you can get it then. Of course, they might not know about it, and that just means the travellers haven't done their research - as any animal like this is going to be widely known and remarked on. So fair enough TPK in that case. But if they have done their research, they will know that this kind of bear exists, and will see signs of it around the cave if they look - scat, old kills, tracks.