The trick to kill The Master Below with this tactic is to have as many “Scroll of Maelstrom” as you can. Having lots of creatures to summon also helps. If you don’t have any Maelstrom scrolls or don’t know where to buy, you can craft them yourself using Skaenbone, Adra Ban and Stelgaer Tooth. You can safely kill the leftmost Adragan without turning hostile with other ones. (It’s probably a bug, since you count as in-combat -so you can chant phrases if you have a chanter in your group)
For the fight itself, be sure to turn The Master Below’s back to your main group. Otherwise her breath attack will obliterate your party. You can use your main Fighter/Tank for soaking that first damage. If you have a Priest with you, 6th level spell “Spark the Souls of the Righteous” also helps.
After starting the fight, debuff the boss with anything you have. (If you have the Scale-Breaker trait from the Dragonslayer NPC be sure to use that too) Then start casting Maelstroms consecutively. 15 or 20 of them should get the job done easily.
It wasn’t a perfect try on my behalf, but as you can see you don’t have to micro your characters much. So it’s a relatively easy way to deal with this fight.