[Aerich]

As I said elsewhere in more detail, one way to get around the thiefless void in difficult areas when dualclassing is to start with 5 characters and add in the thief later on. You could potentially add a thief at the beginning of Dragon's Eye, get up to a decent level by DE lvl 5, deal with all the traps, and then dual over to your class of choice.

There are two positives to this strategy - takes away the admittedly minor XP loss/aggravation of your dual dualclass thief strategy, enables you to free up another character for effective multi or dual classing if you wish, and gives the other five characters a little extra XP bonus because you will only split early XP between 5 instead of 6.