HunterG 10

This one is unlocked by collecting 500 bounties. It's pretty time consuming regardless of how you do it but here's the best advice I can provide. Bring up your fight list (button at bottom right) and go through the opponents looking for those with bounties set on them. Click attack and check the terrain. If it says "very hard", skip it and continue on to the next one. When you get to one rated hard, normal, easy or very easy...attack. Beat them and their bounty is yours.

There are only about 30-50 people on your fight list at any given time and only a small percentage will have bounties on them...and even a smaller percentage will have a terrain that goes in your favor. So once you knock out the ones you think you can handle, here's how to clear your fight list. Click each name one by one and select "set bounty". The bounties can be lowered all the way down to $100 so it barely costs you anything to set bounties on your entire list if necessary. Once you have them all set and there are no remaining names, close the fight list then re-open it. It will refresh with 50 new names. As I said, this is still quite a grind but knowing how to get a fresh set of players on your list as often as you want can be a huge help. Other than that, stockpile as many boons as possible to make wins easier and simply keep on grinding.