Try to keep a medic with every group of party members you have. If you have 6 that you keep together, have a medic be one of them. If you have 12 that you use in groups of 3, make one of those 3 a medic, etc. Also try to have one medic per 8 members regardless of whether they stay together or not.
Normally it is wise to keep the focus of a fight off the medic. He will heal party members much more quickly than any other member of your party, and use less charges of his first aid kits to do so. However it is important the medic can fight if need be, so including him in fights early is a good idea. If the enemy you are fighting turns toward him to start targeting the medic, micromanage him and have him run away. Focus should change fairly quickly, and then bring him back in.
In any fight where you do not think you will win, move all other members of your party together and fight your enemy, and move the medic some distance away. When the rest of your party members are defeated, wait until the enemy moves away and then heal them.