My plan (please don't steal)
[spoiler]Chain Mindrape a group of 20 people to believe that their greatest love is Dispater. I will use Commoners for this purpose.
Cast an Intensified Chained Twinned Repeating Love's Pain on those people. For a moderate assumption to my CL, it would most probably be at least 70, if not more. This means that:
I take large amounts of Intelligence damage; at least 12 points, potentially more. Ouchy. However, Dispater takes 4x(420+19(210)) damage, or 17640 damage, half of which being potentially Vile Damage, across planar boundaries (spell says that distance means nothing). He gets no save, no SR, and since I'm not casting it on him, it bypasses his immunity to Mind Affecting. Dispater has 1648 HP. He's dead, Jim. And I'm pretty sure all of that stuff is from legal books. Please tell me if it isn't.
Dispater dead for the priice of 12 Intelligence damage, a 12th level slot, and a 20th level slot. It's elegant, I tell ya.[/spoiler]
Now, how do I get the spell to have the Good Descriptor? It has the Evil Descriptor, but would adding Consecrate Spell work? It would make the spell [Good] and [Evil], though...