An optimized deep gnome monk generally looks something like this:

Dreadmaster of Bane(12)Illusionist(7)/Thief(1)/Monk(1)
Total level 21, 9 levels free, suffers 20% XP penalty depending on how you level it up

STR 8
DEX 20
CON 10
INT 15
WIS 20
CHA 1

Base AC: 10
DEX: 7 (20 base, +5 from Cat's Grace or Chimandrae's Slippers, -1 from Potion of Action Transference or whatever it's called, from the Ice Temple Battle Square, for 24 total)
WIS: 14 (20 base, +5 from Every God Ring, +2 from Potion of Action Transference, +2 from Bane quest in normal mode, +2 from Bane quest in HoF, +7 from level ups, for 38 total)
Shield spell: 7 (armor, note that Illusionists aren't supposed to be able to cast abjuration spells like Shield, but having another spellcaster level removes the restriction)
Ghost Armor: 5 (deflection)
Dodge: 1
Deep gnome racial bonus: 4
Brazen Bands: 5 (only available with the Collector's Edition merchant you get by purchasing the Avarine Decanter from Nym in the Wandering Village, then freeing it)
Haste: 4
Barkskin: 5
War Chant of the Sith: 2
Expertise: 5
Sunfire Talisman: 3
Swing from the Masts: 3 (thief-only item available in the Underdark)

Total AC: 75

Adding Recitation and Prayer to penalize enemy attack bonuses by -2 and -1 will bring this up to 78. This is 6 better than the mythical 72 AC, which is the minimum amount to force endgame Slayer Knights of Xvim, with their +52 AB, to roll a natural 20 to land a hit.

This build is pretty restrictive, but it offers a little wiggle room in terms of levels and AC. This character can cast Blur, Improved Invisibility, Stoneskin, Blink, Mirror Image, and Heal, all important tanking spells, but otherwise it's good for basically nothing else. Its summons are weak, its damage spells are weak, its fighting skills are weak, and it has no utility skills, but if you add more cleric levels, it will have very strong Symbol of Hopelessness spells.