For a caster, I would go Druid10// Rogue1, Wizard9(or wiz prc's)
now dump str and dex to 3 as mentioned earlier, dump cha if needed
make sure you take natural spell, maybe eschew materials and some metamagic.
an interesting idea would be to take a hawk(or otherwise matching animals, perhaps with improved familiar or shared WS) for animal companion AND familiar, now WS into a hawk(or whatever they are), while flying around casting spells you have an effective 2/3 miss chance (not accounting for true seeing, etc..), hrm added illusions of more birds could be fun too
if you can talk "undecided" into a rouge level, cut that out of the build
EDIT: this may not be Optimal but, looking at the party, you might as well have some fun with it