The best way to do this is to get into a battle, take down all but one guy and let him take you down to less than 5 health. Best to do it early on in story mode because once you get past the first three dungeons, the bandits get stronger and can start throwing knives that can make it more difficult to win.
The main thing is once there is one guy and he has gotten you down to the required less than 5 health, keep dodging by pressing Y whenever the bandit is about to attack to avoid taking any more damage.
You can also check out my solution for
These can be done at the same time, just follow the solution for very brave or very stupid and if you do not have this achievement it you will get it following that solution.