The standard Hollywood version of a bear attack has the bruin on its hind legs, swinging its front paws like a clumsy boxer. Our hero then meets him face-to-face and usually settles the matter by planting a large Bowie knife in the bear’s chest. Real-life bear attacks (and this covers both black and grizzly) are usually sudden, unexpected rushes from cover with the victim on the ground before they even know they are in danger. Reading accounts of these attacks has lead me to conclude about the only defensive tactic you have at that point is to feed one arm to the bear while trying to use whatever weapon you have available with the other. What you never want is for the bear to seize you by the head.
Perhaps the most classic tale of knife-versus-apex predator is that of South African game ranger Harry Wolhuter in 1903. While patrolling the Kruger Wildlife Reserve, he was riding alone in the dark toward a water hole to camp for the night. An African lion suddenly sprung out and pulled him from his horse. With no other option, Wolhuter drew a plain European-style, single-edge butcher’s sticking knife from his belt sheath and stabbed the lion several times behind the shoulder in the general area of the heart. The lion released him, stumbled a few feet, and died.