I thought I contribute my favourite tatic here, people may already use it or it may seem obvious but I thought I say it anyway, presenting the cloak and dagger.
Make sure that your whole party is near their enemy but not in sight of them, make sure that everyone has all the offensive and defensive abilities that any charecter in your party can have are activated. Once you are satisfied haste your party, get your mage to cast mass invisibility on your party. Surround your enemy and then attack as one. I have defeated the twisted rune, celestial fury party and the party in the sewers using this tatic with ease. Even dragons fall to this. I hope this helps anyone.
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Sigmar has only one foe,
and he is Tiax!