How to kick Improved Hive Mother's butt :
SPOILERS
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The last time I did it was with Keldorn - using Crossbow of Speed and Bolts of Biting - he was Improve Hasted -> 7 ApR !
She was also Greater Malisoned and Doomed from other guys !
That poison from Bolts of Biting destroyed her in no time.
The problem with other tactics is that (as Six mentioned) she can heal herself infinite times

, but poison prevents her from doing it.