Here is the full list of all 12 The Last Bounty Hunter trophies - there are 8 bronze, 3 silver and 1 gold.
Complete the game in multiplayer mode
Kill all the bounties in one game
Complete a play through without killing any innocents
Finish the game ( single player )
Capture all the bounties in one game
Kill or capture Cactus Kid
Win a showdown
Kill or capture El Loco
Kill or capture Nasty Dan
Shoot twelve enemies in a row without missing or reloading
Kill or capture Handsome Harry
Shoot six enemies in a row without missing or reloading