Thanks for all the save files I now have platinum. Thought I would help out with this one since it took a while to finish.
Here is my save file right before the end of act 3! New link:
https://www.mediafire.com/file/oahovauf4y0moce/CUSA00054.zip
I got it as close to the end of act 3 as you can, it would have been even sooner except loading the saves always puts you back in the camp.
Just load it up, they give you a brief demonstration on how to use the Thunderpoon. Drive to the mission marked on the map and finish it by doing the following steps.
Use the thunderpoon on the reinforced outer gate.
Use the regular harpoon to pull down the small red towers shooting at you
Drive back to the entrance (so its away from all the snipers) and get out of your car and mash square to finish off all the small fry
Drive back over to the jaws entrance and use the thunderpoon on the snipers (your ammo regenerates and they are next to explosive barrels)
Destroy the door with the thunderpoon and pull it down with the harpoon
You will then get the trophy
Sorry its a bit of a sh*t mission but its as close to the end of act 3 as it gets.
Also if anyone gets stuck on Road Warrior on the original save pack (slot 2), you can get it very quickly by fast traveling to Pink Eye and buying the Lumber pile upgrade and you instantly get enough Griffa Tokens to buy Road Warrior at Griffa.