Alright! After 94 hours of playtime I finally did it. But I could have very well ended up again getting Astarte killed third time in a row. The thing that worked from me was I had a summoned skeleton pretty close to where the boss spawns and he took 2 of the 4 spells at start - which meant that Astarte lived!
Mortal - my advice would be the same. Have someone pretty close to the boss spawn but in opposite direction to Astarte so that at least 1 of his spell doesn't hit her.
