Ok, how about this - beholder rays going thru walls in Terminal Delirium? We had these. Hitting for 1.6k damage on a defensively geared, specced and tank based pally. With Deathblock and Deathward actve and Beholder Optic Nerves still not depleted. Now tell me about tactics.
The usual tactic for beholders is ranging, pulling or evading. Evading is calculated against the DC of the spell / effect. Our lv. 23 rogue is a skilled evasionist (monk splashed, stick monkey, high reflexes) and even that rogue had a hard time evading the champ rays on HEROIC ELITE. Terminal Delirium does not give lanes long enough to hit the beholders with long range spells or ranged weapons w/o getting hit by antimagic and their rays. So basically this limits the tactics that might be applied. We tried several approaches w/o success. Our approaches had been: Ranging (lanes too short, we had been in range of beholders rays), pulling (being especially problematic with the traps in the back) into BBs and firewalls, Assault (hitting hard as fast as possible), tanking (not adviseable, since the tank = my pally got rayshooted within seconds). Yeah, with enough patience and resources we would have made it past them - but where is the fun there? I am no masochist and my group mates are neither. And I prefer not to summon altars of repair or using up my stack of shards for repairs. And that´s exactly what happens - you have to wear down the beholders in there, getting killed several times.
We completed the quest on EN easily by the way. No champs, only a few beholders and these being evadeable and saveable on default. The quest is lv. 26, we did it with our lv. 23 toons and had no problems at all. I am looking forward on EH today. After all I have seen that will be a good run - and will be much easier than the same quest on Heroic Elite.