To add some extra pointers to the @rockinredninga's solution:
- Simply throwing shields on other brawlers who also hold a shield won't work as most of the time they will deflect the shield. To make things easier, equip a handgun with armor-piercing ammo. Shoot at their shield till it breaks, then attack them with your shield.
- Having the "Leap of faith" perk (it's basically a double jump) will help to jump around and avoid other brawlers attack you
. Bonecrushers pit will always have 7-8 brawlers for you to churn this achievement, even after you finish the main game.
- It also helps to have "More lung capacity" perk, so that you can keep running around the pit to avoid being hit by brawlers. You can also set the difficulty to "Explore" and make things a little easier for yourself.
- If you simply jump into the Bonecrushers pit, you will be attacked by all the brawler's together which will make the process tedious. Here's what you can do: Kill one brawler at the start of the process from one of the rooftops of the pit, this will alert other brawlers and they all will head to the exit so that they can reach you to attack you. Since it is a narrow exit, they can't all exit together. Wait until only one or two bralwers remain in the pit and then jump into the pit. Now the brawlers will come at you at a much more manageable rate.
Good luck, hope these tips would make it much less irritable for you to get this achievement.