To unlock the Grin and Bear it trophy in Red Dead Redemption 2, you need to survive a direct assault from a bear and then kill it, 18 times over. If you come across a bear and it gets a hold of you, it will pin you to the ground. You can mash
to get out of it, and then you will have a small window to shoot the bear repeatedly before a second lethal attack. Hit the weak points and you should be fine (It's easier to do this in the late game once you have the weak points available in Dead Eye).
You can attempt to rack up 18 attacks randomly; you'll find bears in Tall Trees as well as across the Grizzlies and around Wallace Station. You can also save scum -- according to some, you can survive one bear attack, leave the area, save on a new slot, delete the old save and reload the latest save to get a respawn. I wasn't able to replicate this perfectly every time but it certainly worked a few times.
ImmortanProstate adds: "... the kill will only count if you shoot the bear while on your back post stabbing the bear in the neck and hitting the shoot cue. In order to make sure you don't miss this, spam R2 immediately after the stabbing sequence and you will get a head shot and the kill. If you miss the cue then you will end up shooting it multiple times in the torso till it dies or the character stands up and the bear or you die and that is how you know if you didn't get the registered kill for this trophy. Had to shoot four more bears than necessary to pop this trophy."