How would the creature be vs. death spells?
My personal strategy against such a creature would be:
1. Spot Beastie before he spots you. (+30 Spot enhancing item would be preferable)
2. Cast Haste
3. Quickened Enervation (Touch AC: 12)
4. Partial Action: Double Empowered Enervation
5. Wail of the Banshee
On average, the saving throws will drop by 7,5, which gives +19 on fortitude saves.
And hope that the beastie will fail it's saving throw, which is feasible with DC 36 vs. a 9th level spell.
Conclusion: The beast will have to roll 17 or better to survive, and that vs. a 18th level wizard. That's 20% chance. Of course, if it survives, it will have to kill the wizard in one round, or he'll throw another wail of the banshee in its face.
Optionally, if the PC has that continual haste on himself, he only have to win initiative, which he will.
I don't know if this strategy would work, but off the top of my head, this is what I would do.
With party members distracting the beast, I think it would be easy to pull off.