The unseeing eye quest was the toughest of the 4 for me, though that may be because my party was at a relatively low level at the time. Either way, it was hard-later I discovered, as has been pointed out, that fire elementals seem to be the racial enemies of beholders
, and cut them down in swathes. Windspear hills would be a close second, with Planar sphere and Umar hills bringing up the rear.