Quarion, you missed my thread on this subject which I think has some decent ideas contained in it although it's not gotten as many responses as some others. Here it is:
http://forums.ddo.com/showthread.php?t=125085
(I hope I did that right, never linked anything before)
Anyway, in the meantime, here here are my own (slightly edited for context) posts from the thread:
I'm writing this after reading in the general forums that certain shadows have been given a blanket immunity to wall of fire (presumably & co.) for balance reasons. Like many players, I am inclined to call this change, for lack of a better term, "cheese."
Going forward, it seems simple to me that you should have your NPCs fight magic with magic. Why should those shadows need to be immune to fire? Aren't their high CR vampires in the quest? Have them cast Break Enchantment on our Walls of Fire, or throw up a Globe of Invulnerability when engaged. The fights will be more difficult, and you won't have an angry player base accusing you of cheating.
That's not enough? No problem. Code your Walls of Fire as targettable entities (like Flaming Sphere) and make them extinguishable by things like Ice Storm or Cone of Cold.
I understand that the quest was far too easy with Max/empowered wall of fire (as is the case in a great many places, btw), but your solution is a poor one. In fact, in retrospect, I think your decision to put Wall of Fire into the game was likely a poor one. It is simply too powerful relative to other spells, and borders often on game breaking, but I'm sure it's too late to remove it now, so do something interesting to bring it in line - make it counter-able - as opposed to just tossing out immunities where none should exist.
Beyond that, give some thought to the (often repeated) suggestion that mobs be scripted to move out of damaging lingering AOEs. There need only be distinction between effects on a mob between environmental effects and applied duration effects. IE whenever a mob takes dmg, if it's source is tagged as "environmental" (a la Wall of Fire) it will make an effort to move out of it, or at least have that affect its behaviour in some manner (ie, Ouch it burns here! I'm gonna whack that other guy over there instead).
I really hope that you guys read this and consider, if not these, then some other options for dealing with this sort of problem.
I just want to point out that I'm not saying any of these are perfect as stated, but rather trying to encourage discussion of viable options to "make them immune."
I agree with those of you who say we can't just give them a simple "it burns get out" script, for the reasons you posted. However, as sad as it is, I think that would probably be better than what we have now - which is of course - "Ow that hurts, I'll stand here and die in seconds, to no good effect." At least, if they ran out of it and we had to kill them with range, it would take longer - haha. What I would suggest is a slightly more complex script that would have the mobs react differently, perhaps based on archetype. Casters would move out and attempt to dispel 'environment' effects. Mellees would shift agro based on relative distance from the "Owie," and rangers would of course retreat to range, as they more or less do.
Other options exist too of course. For example, Metamagic enhancements could be disabled for Wall of Fire. As the spell, per PnP is effectively a form of crowd control and not a damage dealing magic, this could be justified I think. Maybe in this case make it into an object again like a Flaming Sphere, or something, and have it 'pulse' an antipathy effect which the mobs would have to save against in order to build the nerve to pass through.
Just some more thoughts, do you agree? disagree? Come on guys, let's give the devs some conversation here, because it's pretty obvious by now that Max/emp walls of fire are getting out of control.