The blind man's bluff, eh? Suo can handle invisbility, worst comes to worst. With Shiho around to help train him in the eyes of the mind, he'll have plenty of practice for this. A balanced blade is a perfect instrument, one that can find a hole even where only empty air appears. While he may struggle to get past Imoen's magical tricks, once he hits the fight will end fast - Imoen may have a thousand spells at her disposal, but she's still a DnD mage, and dies as fast as defenseless sheep once you start landing deadly blows.