First, you must get the "Transfer Supplies" ability under the medic tab. You must be RANK 2 in order to get it. Now, what this ability basically does is transfer your supplies to a friend at the cost or your own supplies. Once in a battle, buff everyone's health up until you only have one more [pip] supply left. Then press left on the D-pad and hold X near an ally to give the rest of your supplies to him/her.
Cheers!