Direbear have little health compared to torpor. Keep a few tamed dodos at your ramp trap. When you've shot the bear enough for it to be bloody, throw in a dodo. It will instantly kill and eat it and start regenerating health. Give it a moment or two and continue darting. Happy hunting!
Dire bears have the same base torpor and torpor gain per level as an allosaurus, meaning darts are almost a requirement (unless of course narco traps are utilised). To put this in perspective, Allos have 63% more base health than a dire bear and gain more health per level, and even with a lvl 150 allo there is still a 1.5% chance that they’ll die for a 100% damage Crossbow. In summary, use darts, it makes it a hell of a lot easier.