After being returned to Easthaven and entering the Ice Tower, I am confronting "Pomab" (doubt if spelling is correct), in a room full of mirrors. Lightning is cast or triggered, and my party is protected from lightning, however, "Pomab" keeps multiplying and Crystal Guards seem to have WAY too many hit points. What have I missed?

First, don't bother trying to beat down the crystal guards. Have a couple tanks keep them busy. Maybe summon some allies to help. Otherwise have the remainder of your party EACH attack a different Ponab with ranged weapons. Eventually you'll get the right one.

Are you sure Mag? I did exactly what you said, but iot didn't work, but in the end it seemed like after a period of time, Pomab starts cursing as if his power was fading and he became ''one''.

I ignored Pomab and wiped out the Crystal Guards, then killed Pomab rather quickly. Hasted, blessed, etc

The crystal guards are extremely hard to kill. The "correct" colution is to cast protection from lightning (essential) and haste on your party and concentrate your attacks on Pomab.
Summon monsters to help keep the crystal thingies busy. Missile attacks won't work on Pomab, you have to attack using melee weapons. The various copies keep teleporting around the room, so haste will help you chase after him.
Eventually all the false copies will be "killed". The last Pomab standing is the real one.
Kill him, and the crystal thingamabobs will also die.

Hayashi is right ... focus on the "Pomabs" only. The other creatures will die when the final real Pomab is killed. Consider using the following spells to your pre-combat preparations:

Use potions of strength / invincibility / heroism etc first as they are the longest lived.

Protection from evil 10" radius
Hope
Courage
Prayer
Recitation
Protection from Lightning
Righteous Wrath of the Faithful
Haste (this is the second to last spell you should cast)
Defensive Harmony (this is the LAST spell you cast before entering the room because it is VERY short duration)

Have Clerics/Druids/Paladins cast Entropy shield
Have Mages cast Stoneskin / Mirror Image

Then go into the room and kick Pomab's butt!