Getting drained -20/-30 Con every second and then getting stunned is just simply a stupid mechanic, not fun, impossible to avoid by moving around the beholder or any tactic (you know, like avoiding the Anti-magic field requires you to stay BEHIND normal beholders).
Is it WAI to get spammed with -30 CON every second? Because if so, good job at making a rather fun and hard quest on heroic, one of the most annoying end fight ever.