Just did spawn to remind myself on Challenging. If anyone was wondering - much easier in Turn Based.
1. Cast no buffs at all, summon nothing - very important.
2. Have Octavia or another ray-caster nuke Xelliren to initiate combat with a maximised Hellfire. This will kill in one.
3. Valerie into combat with Tyrant (stacking -1AC per hit)
4. Pet into combat to soak up a couple of blows.
5. Tristian in reserve with a book full of Cure Critical,Mass, and Heal, Mass - cast when necessary otherwise do nothing.
6. Jubilost to Fast Holy Bomb
7. Druid and MC twiddle thumbs within range of Tristian heals.
8. Grandmastered Hellfires x3 + Holy bombs plus some hits from Valerie is enough to kill.