Red Dead Redemption 2

This can only be done from Chapter 6 onwards. I'll avoid any spoilers in the following guide, but it's basically just following a mission strand.

Head to the park in Saint Denis with the large pond, just West of the train station. As long as you're not wanted and haven't just been shooting up the place, there should be a gentleman with a little submarine (or as Arthur says, "A little toy boat!").

Talk to him and complete the mission. Once completed, head to a hotel or out of Saint Denis and sleep for 24 hours. His next mission should be tagged but if not, it's North-West of Brandywine Drop, near Annesburg., marked by a little circle and rectangle on the map.

Head there, do his mission and leave again. Leave the area, like to Saint Denis or Valentine and sleep for another day or so. 24 to 48 hours should do the trick. Now go back to lab in Doverhill.

Once you've done all this, it should pop.

If you're trying to hunt all the achievements in this game, I recommend coupling this one with Skin Deep/Zoologist, as you'll naturally come across bears while roaming through nature. Listen out if you can for their roars and rumbles. If you're just looking for bears, there are lots of guides were to find them, but it is really luck of the draw. I had pretty good luck at Wallace Station (map here. Big thanks to /u/Mangeloo.)

Now onto the actual attack. Each one will count once the bear attacks, you get free and then kill the bear while still on your back. Make sure you have a decent repeater or revolver/pistol. When you see the bear, try to get on some flat, even flooring and avoid slopes. The bear may charge but not attack if you're standing still so very gently move to the side while the bear charges you. If all goes well, the bear will pounce and start to attack. You'll see the "Break Free" prompt, so mash it when you can. Once the bear backs off, you'll take out whatever weapon you had equipped. Just keep firing until the bear dies. If you stand up, the kill won't count, so don't try to move.

You can skin the bear for meat and a poor pelt if you'd like, but make sure you leave the area and sleep (aim for 24hrs if you can) and then return to the area, if you're trying to get more to spawn.