Nishruus are not meant to eat magical items.
The guy who made the Baldurdash Fixpack is one of the game designers, and whatever is in the Fixpack, is what the Official Patch
fail to fix.
To play without the Fixpack, is to play a crippled game. What Nishruus were meant to do is to
drain magical items of their charges, not wholesale destroy any one of them. One weird bug that resulted from item consumption was that sometimes an item gets thousands of charges because the numbers roll over as the Nishruu happily "drain" it.
Nishruus cannot be compared to Rust Monsters, as they do not serve similar purposes.
As for how your Enchanter will fare; quite well in my opinion, since... you are not playing a modded game.
Once you get the Staff of all Staffs, the game difficulty takes a nose dive. Are you familiar with all the things that clones can do? Alson's Spell Reference Website will prove an invaluble read.
It will not be a cakewalk compared to a Generalist Mage, but because you still have access to Illusion spells, you will encounter little difficulty. Truth be told, I cannot think of any non-modded enemy that will pose a serious problem for your Enchanter.
To deal with Critical Hits, get the King of Strolm Mask( Windspear Hills).
Rings I recommend will be the Ring of Acuity and Ring of Wizardry( Planar Sphere( Quest, Stronghold Quest))( Ring of Gaxx is another option).
For Robes, a Robe of ArchMagi.
Staff of Magi and the +4 enchanted sling found in the Cult of the Unseeing Eye Quest( +3 enchanted sling can be found in the Umar Hills Quest).
Hoard Sunstone Bullets, they are +4 enchanted.
Boots of Speed are the best of course, for hit&run spellcasting.
Amulet of Power needs no explanation
Belt of Inertial Barrier( keep a STR girdle too).
Cloak of Protection +2( Cloak of the Sewers until you get the CoP +2)
Bracers of Defence AC 3