I made her a battle wizard of level one because allowance must be made for a tabletop. There are no options for magic of level lower than battle magic and writing a lot of special rules for the character just isn't right. I was choosing between low end battle magic and scout/infiltrate (giving both is seriosly OP) and chose the former. Dropping magic in favor of scout/infiltrate is another option.Not very. She's a hilariously overpriced, somewhat effective tarpit. Without battlespells, she just doesn't have the stat line to seriously threaten anything. With the seed and talisman she's pretty hard to take down, but since she only has one attack (and effective S5,WS4), she won't be killing anything important anytime soon. And even a block of standard units will grind her down soon enough.
I'd probably give her the scout or infiltrate special rules as well. That way, she becomes a decent artillery (including mages) hunter, especially with the high mobility of shadow steed (that may or may not ignore terrain tests, I don't know. It would fit the quest anyway). So now the enemy actually has to deal with her, and since she's pretty though, they'd have to spend a lot of killing power on that. Which means the main army doesn't take such a beating.
With that in mind Mathilde still is ovepriced but on her shadow steed she is that pesky debuffer you just can't get rid of. Your units are too slow, or too weak, or too expensive to rekill her times and times again (remember, our Seed allows for battlefield recharge), and she's got Terror and a variant of Rune of Spite to erode your army.