Not really a full guide just some simple tips to help you unlock this achievement.
In order to unlock the challenges you have to find Scarlett scattered through out the episodes. There are 20 in total.
Here is an excellent location guide by Quantum Binman that you can follow:
http://www.xbox360achievements.org/forum/showthread.php?t=51...If your not upgraded some challenges may seem tough. What you want to do is wait till you finish your first playthrough. At that point save up about $25m and purchase ulti-mondo (which gives you more strength) and dark booster (which gives you unlimited blood). Those 2 upgrades unlock after your first play through.
With those two upgrades the challenges become infinitely easier. You will be getting AAA ranks with out even trying.
You can also set the difficulty to easy if you like.
If you need tips on making money check out another great guide by Quantum Binman here:
Hope this helps.
Any helpful tips are welcome.