As I said, a triple 'should' work without HWW.
20d8*3, ~270 damage on average (20*4.5*3). in half it's 135 (Assuming a save).
Assuming the level 20 cap is correct on that heh.
Just need to prolly toss a few Lower Reses first (Triple Lower Res Spell Trigger should bamf out Solaufein's huge SR). Should on average kill Solaufein.
If it doesn't, then a Time Stop Quad Wilting would clear them out most likly.
Negative Plane Protection does work. However, she shifts her target to someone without it if possible.
After 'this life is mine' it tends to go really well for me (her buffs have either faded or got dispelled). But then again I play on normal
.