So, wouldn't a tarpit with enough wounds between models to absorb the rune retaliation and resistant/immune to terror be a better match? Say, a horde of zombies on a sunny day?
More expensive tarpit that can actually do damage may do the job, but zombies not so much. It will go about as well for them as zombies trying to kill Lord in TWW. They hit Mathilde on 5+ and wound on 6+ and don't lower her armor. If they are in horde, they make 9 attacks per turn, but even without horse that 9 attacks result only in
0.25 wounds per round on average, and with horse it's
0.125 with very negligible chance to remove a horse. Mathilde, on the other hand, will hit on 3+ and wound on 2+, that's 1,67 wound from the sword. If we average tendrils over all possibilities we'll get 1,11 wounds against unarmored T3. That means Mathilde usually deals 2.5-3 wounds per combat round. On combat resolution it is compensated by zombies having +3 from extra ranks. Most likely results:
1) Mathilde slowly kills them over several turns, when they become unable to maintain 3 extra ranks they start to disintegrate faster and faster due to being Unstable. (when they lose CR, they lose that much extra zombies)
2) Mathilde has an unlucky round, loses CR and then rolls badly on Ld test and flees. Zombies are to slow to pursue and wipe her out, so she will reform and do something else (or charge them again, but why would she do that?)
Them killing her is not in the cards.
TL;DR: her normal defences (WS, toughness and armor) may not be up to snuff against the likes of bloodthirsters or hero-killers, but they are more than enough to kill chaff for rounds and rounds.