Need help with Assassin optimization

[Fax Celestis]

Rogue 5/Spellthief 1/Assassin 1/Unseen Seer 10/Uncanny Trickster 3, and take the Master Spellthief feat.

Spellthief is in C. Adventurer, Unseen Seer is in C. Mage, and Uncanny Trickster is in C. Scoundrel. Focus the Unseen Seer "+1 spellcasting" on Assassin and Uncanny Trickster "+1 existing" on Rogue.

You'll end up with BAB +13, +10d6 Sneak Attack, full Assassin casting, and the ability to steal 7th level spells (via Spellthief and the Master Spellthief feat). It's not a bad build. I have better, but you're not allowed ToB and it's really really early late.

Another good call is Rogue/Assassin/Unseen Seer/Abjurant Champion, mix and match to suit. It's a bit more frontliney than the above build.