No you don't have to get the no losses on the row to get the medal, getting them over time is fine. I find my Inquisitors are rarely attacked, when they are they use their heal on themselves. Paladins can revive themselves and everyone adjacent.
There are several spells worth getting Phantom will allow you to copy a stack of Paladins/Inquisitors/Rune-Mages, all of which will then be able to heal your units. Aternatively the Ressurection spells directly returns units. As a Warrior however it will be lacking in power, so Phantom will be better for you.
You certainly don't need to restart. Unless you are trying never to lose a unit in the whole game. I'd play it normally first without such a crazy challenge!
Daniel.