This is a two controller method for running through boss rush on Chaos II and Chaos V difficulty.
I do recommend having a character with a minimum of 100k damage as you can see in the video below that if it is much lower it could possibly take a long time and also make sure you have a hefty amount of defense just incase. I completed this with 112k damage and 584k defense.
Text Guide:
1) Have another account (Xbox Live or Local) with a character that has Boss Rush mode available.
2) Using that account create a new character and run through prologue.
3) Using that account go to "manage players".
4) Add Player
5) Select the account you want the achievements for.
6) Select your primary character.
7) Have secondary account start a boss rush on proper Chaos difficulty.
8) Simply kill the boss or bosses. (If going for Chaos Hunting, only need to defeat first three bosses.)
9) Boom enjoy the achievements!
Video Guide: