If you really want to go unseen seer then wizard is the way to go. Beguilers just don't have any spells to use your sneak attack with. You want to be useful in combat right? Well Wizard unseen seer is pretty amazing at making the enemies fall down*

Grab Fire Shuriken and telekinesis. Fire shuriken is a great delivery system for sneak attack and telekinesis is probably the best Sneak attack delivery system in the entire game.

Use advanced learning to get hunters eye so you can gain sneak attack die equal to 1/3 your caster level. Then take the crave feat to add your level to your sneak attack damage.

So at level 20 with rogue 1/wizard 9/unseen seer 10 you can do 165d6+300 damage with just a single casting of hunters eye and telekinesis.






*Provided that the enemies aren't better casters than you and also aren't immune to Precision Damage