Hi ComradeP,
if only you would've seen my complaints about the same thing during beta testing - until I noticed that already at level 3, Men-At-Arms are surprisingly resilient...
Well, unless I missed that I can replace auxiliary units with core units during the deployment stage, the units in front of Fabel always take some losses I'd say. Like I said, I restarted several times and the defenders of the fortified encampment were always destroyed with ease.
It didn't really matter for this scenario, but after completing the campaigns I was somewhat annoyed that the design of some scenarios made achieving secondary objectives impossible.
SSG tester
WitE Alpha tester
Panzer Corps Beta tester
Unity of Command scenario designer
Buying a unit like a priest or the Elven magic user is a good idea. However, you shouldn't do it too late in the campaign or too early. Early on, you need every combat unit you can get your hands on and healing is less important. As enemy strength, levels and skills increase, you'll need healers or units that can do a lot of damage per turn.
The priest or magic users won't be all that useful initially, the Elven magic users more so than the human ones in my opinion as they start with some reasonable temporary defensive/offensive bonus spells. After gaining one or two experience levels, their spell selection improves substantially and they get spells like mass healing (heals all adjacent units to the hex it's cast on instead of just a single unit), offensive area of effect spells and better temporary defensive/offensive bonus spells.
The Priests and Elven magic users are also the only unit where I'd spend all the upgrade points on defence, as otherwise they are likely to die when attacked. Their damage output, if any, comes primarily from spells.
SSG tester
WitE Alpha tester
Panzer Corps Beta tester
Unity of Command scenario designer