Is your medic actually off by itself fighting the battles? Once it's in a stack, it's almost never the defender.
I used to get frustrated with getting stuck at 25XP also. My go-to medic is an Explorer with Combat I, Medic I/II/III, and Morale. Barring AGG or CHA, that requires 26XP, but my supermedic was usually whatever Scout survived early animal encounters and made it up to the 5XP cap. (Attach a GG for 20 more, then upgraded at Compass.) But since I would avoid having him fight (being my medic and all) he would never get to 26 for Morale. I eventually learned to have him wander around and look for a barb Warrior and try to sucker him into attacking on a forested hill. If he lived, he's got at least the 6XP needed for later. If he died- meh, make another Scout.