This has two basic requirements:
1. Have a class with guard
2. Have a class that can heal
This 'exploit' requires that you been in the Huttball warzone. Guard your healer and have him stand in the green pool of poison water, healer then spam heals himself while the tank takes absolutely no damage via the guard, yes still receives the rewards. This is the easiest way to get the following medals for almost no work at all:
Guardian Medal: prevent 2k damage in one life
Shield Medal: prevent 5k damage
Paladin Medal: prevent 10k damage in one life
Protector Medal: prevent 50k damage
I call this an exploit because guard is intended to damage the tank that is guarding his ally as well.
Edit: In addition, your healer is attaining his medals at the same time while helping you. The ones that he can work on are:
2.5k single heal
5k single heal
75k healing