You probably already found out that you can't unequip the brass knuckles and only have one save game.

Please read and act carefully, don't blame me if you mess up your savegame.
All steps can be done in online mode, no need to get offline or create an offline savegame.

Step one: Find the savegame on your computer and make a backup.
It should be under Local Files\Ubisoft\Ubisoft Game Launcher, something like that. The files have numbers. If you can't find Farcry5, just copy the whole savegames folder somewhere safe.

Step two: Start Farcry5. Now the hard part, start a new game (not new game +). I had 250 hours of gameplay at that time, well.... it's a "Leap of Faith". Don't worry, the whole process is done in two hours or less.
Play the story until you can move freely on the map. Now get enough bliss oil and prairie fire to craft at least on dose of "The Furious", that one-hit-one-kill homeopathic (it's not a drug).
Best chance to meet a bull in a not to dangerous environment, is the farm north west of Fall's End.
It could be helpful, to have a dose of "The Fast" ready, because as soon as you take "the Furious", the bull starts running away. It actually worked, achievement unlocked. You should check it before you move on to the last step, or try again.
This technique works also with the bison.

Step three: Save and quit the game. Now you get the logout message that your cloudsaves are synchronized and more importantly a short message that the achievements are saved to the cloud. Since the achievements are bound to your Uplay account, they are safe now.
All you have to do now is to copy your backup savegame back into the Ubisoft Game Launcher or Farcry5 folder, whatever method you used before.
Now you can start the game again (online mode). You get a message that your savegames are not in sync, and you have to choose which files you want to use to sync the save files. You want to save your old files from the hard drive of course and not the cloud save.

Doesn't hurt to keep the backup, by the way.

I hope this helps. See you in Hope County.