The topic has already be solved :/
But next time, please give more informations:
1. How much HD does it have?
2. What creature type does it have?
3. Do we have only two spell lists or do we can use feats or own builds?
4. What about equipment?

If we can use our own build, just attacking it 1,000,000 times should work just fine.
If it has only 1 HD, wish yourself into a vampire and make a negative level - without save.
If it has more HD but not too much, you could Chaos Shuffle your feats into metamagic ones and then go for Maximized Repeated Twinned Split Enervation (lvl16) Quickened Repeated Maximized Split Enervation (lvl16), next round cast Maximized Empowered Twinned Split Enervation (lvl 14) and Quickened Twinned Maximized Split Enervation (lvl17). That is kinda heavy but with a bit optimization it works just fine to fill those highly metamagic'd spells into ur spell slots (i.e. Celerity is a free Quicken). It would be 88 negative levels.

I'm not quiet sure about this, but I think Irresistable Spell cheated around that SR should also work.

Another thing: Vorpal.