So, whats the problems with this spell that I don´t see??
First, as noted, the likelihood that a PC will have M's Disjunction ready to fire is pretty slim. Secondly, in order to counter Elminster's Evasion, they'll need to actually know what it is and that he has it active all of the time, which is definitely in the realm of
player knowledge as opposed to
character knowledge.
That said, let's assume that the PCs somehow manage to secretly surveil Elminster for weeks on end without the sage noticing and learn his most intimate of routines, up to and including the application method of his most secret evasion spell. Elminster has a
lot of other bennies. By virtue of his nature as a Chosen of Mystra, he is immune to:
* Aging
* Disease
* Disintegration
* Poison
* Sleep
He may also call upon the Silver Fire of Mystra for any one of these effects at a time, once per round, without using an action. All effects are as if cast by a 20th-level sorcerer, where applicable:
* Duplicate the effect of a Ring of Warmth.
* Duplicate the effect of a Ring of Mind Shielding.
* Banish all external magical compulsions upon the wearer as per Greater Dispelling.
* Function without food or drink for up to 7 consecutive days.
* Dispel a dead magic zone or antimagic shell, permanently (this effect may be used only once every 70 minutes).
Add to that his 26-ish levels as a Magic User and various levels in other classes. Even
if PCs do manage to er. . . disjunct. . . him, they'll have a
very difficult fight on their hands. Of course, if Elminster never becomes a Chosen and never sleeps with Mystra, he loses all of the aformentioned extra benefits and becomes a fairly standard character (albeit one with roughly 30 character levels).
This Elminster would
definitely be a departure from the one we have now. I envision such an Elminster as somebody who had to learn hard lessons, scraping his fingers to the bone in his youth, and learning his trade via the school of hard knocks. This Elminster would be more Harry Dresden and less Albus Dumbledore.