Seems like when they copy skills they use all skill minimums at 100% efficiency since monsters can't fail spells. For example, if you have 70 chiv, a changeling will cast Holy Light (55) a lot. If you tried to cast Holy Light @ 70 chiv you'd fail most of the time, but the changeling won't. If you have 100 necro (sampire) the changeling will often use Strangle (65). You are, indeed, your worst enemy.
I'm not sure how changelings gain armor ratings. Their natural form is definitely weaker. If you can sick a dragon on one and then invis yourself, you'll end up with a dead changeling much faster than if you allow the changeling to change form. I'd assume a stealth tamer is probably the best for the Weald in general.