It's usually a good idea to have 2 clerics on your team, so you won't be lacking a good healer if one gets knocked out. If both get knocked out...well if that happens, the enemy is definitely too strong for you. The other two members of your party should be a ranger and a mage/conjuror.
heh, thought my defenses were unbreakable once, and then I got hit by arty which knocked out all but three of my soldiers. Then the Germans swarmed me.