Bag of Bones
Deliver 50 skulls in one score in Greed against adept or higher skilled bots.

This achievement can be annoying or stressful because bots do not cooperate. I discovered you can get around this annoyance with the help of another player.

Start a 1 v 1 game against one adept difficulty bot. Set the maximum amount of players/bots to 2. I also recommend using the speed freak and instagib mutators to speed things up even faster. Turn on "forced respawn" so the helping player doesn't have to press anything to respawn.

I recommend playing on the facing worlds map since you can farm kills very quickly for the 50 skulls. The enemy player will always spawn at the back of the map in the same two places which will be right next to where you previously killed them.

After the game has started, have another player join the game, they will replace the adept bot. Kill them over and over until you have 50 skulls collected. Then go stand next to the capture point where you submit the skulls for points. Have the other player disconnect, an adept bot will rejoin the game. Wait a few seconds for them to spawn in. Then walk over the capture point before the bot catches you. The achievement will unlock.

This method takes around 5 minutes or less for each player, and if you are working with one for the coop achievements anyway, you should get this achievement out of the way as well as it will save you time and effort.

Additional Note: Occasionally a skull will land on the terrain in a way that makes it difficult or impossible to pick up. This is very rare and happened probably less than 10 times over the course of 500 skulls. Just ignore those and keep going.