[Admiral_Aorta]

If its astral levels haven't been boosted too much you could try massed magic duels, one of them will likely kill it eventually. Shatter does a lot of damage to lifeless creatures, but that might still not be enough to kill a sphinx. Instant death spells with penetration boosters might work if you use enough of them, even with high magic resistance. If you're a blood sac nation then you'd probably do better to go for a dom kill rather than trying to kill it directly.