(( Fidelis,
I can see that. However, that's a fault of the rule-writing than my suggestion or interpretation. I'm pretty sure I'm not the first person who noticed you didn't have to target a Lord or Hero model with how the rule is writen.
Overall, my understanding consensus is that the Witch Hunter was meant to hunt Characters, id est Heroes and Lords. But, the rules for him just state "model"... if the rule stated "Lord or Hero", there would be no loophole.
Anyway, this brings up a question to me. Magic Resistance says "...bonus to its ward saves when saving against damage cause by spells...". So, if I have a unit with a Witch Hunter receive hits from "Flaming Cloak", "Shield of Thorns", or other augment-effects that cause wounds in combat, do you get the Ward Save against those? -- My thought would be that the Magic Resistance does offer protection from these.
For that matter, if my unit has a Witch Hunter, and it receives wounds from an enemy unit Augmented with something like "Flaming Sword of Ruin" or "Enchanted Blades of Albian" where the effect makes the attacks magical, do you get a Ward Save from that? -- My thought is that in this case you could make the argument that the damage is being caused by the spell... which is rules-bendy.
What about if a creature's attacks are inherently magical... for example, from Hex Wraiths? Or Dwarven Runes where the effect makes the attack magical-- My thought is that the magic effect isn't spell damage, so you shouldn't get it.))