They are infinitely annoying enough already with a high initiative. No unit you own, human or elf, will ever act before a Mist Walker. A level 3 Mist Walker debuffs you for THREE rounds. Pair that with their high movement (4), a Mist Walker in Slot 1 can reach
any of your units on opening round, bearing no obstruction, so you have to sit there and take a debuff like a sitting duck. Then they gave her ridiculously high health points too! My level 3 archer has a +20% attack bonus buff (on top of my Needles and fed Fire Phoenix) and it still takes 4 archers to kill one Mist Walker. That's just nonsensical. And that was just at Province 21.
They're not as effective if secondary opponents are mages. They take a lot of damage and do little damage back. They work better if you pair against Light Range enemy and secondary opponents are Light Melee. In manual fights, I'll also use them if secondary opponent is Heavy Melee, but it's dicey on autofight because AI will send them to face a Heavy Melee and that will be the end of them. Manual fight, I can keep them away.