I have a Drow Fighter/Rouge in my party who is the guy with maximum AC. Here is how:

Base AC: 10
Armour : 6 (Zuvenbie)
Sheild : 4 (Sad Giant's Cap)
Deflection : 3 (Ring of Protection +3)
Dexterity : 5
Expertise : 5
Generic : 8 (1-Dodge, 1-Swing from the Masts, 1-Flame Dance Talisman, 5-Brazen Bands)

Total : 41

What spells can I use on him to increase his AC?
What types of AC do specific spells increase?

I recommend you read the JUPP (Jukka's Ultimate Powergaming Party), availible here on SP, for pointers to make characters with 70+ AC.

@Shrikanth

Yes, JUPP will really lay it out for you. Given your character, the following spells will help:

-Ghost Armor (Wiz/Sorc) will increase your Deflection to +5
-Shield (Wiz/Sorc; self cast only) spell gives +7 armor bonus
-Barkskin (Druid) will give +3-+5 generic (depends on level of Druid; +5 comes at level 12 or higher)
-Haste (Wiz/Sorc) gives a +4 generic

Assuming you have a Druid of level 12+, the above spells will increase your current AC by an additional +12 (net) giving you an AC of 53. Since you are not a Sorcerer or Wizard, you cannot cast Shield but you can always try and buy a Shield amulet or ring. If you are going into HOF, that AC should let you survive Targos, barely.

Well the guy is a Wizard, but he hasn't got a level up since his first.
I'm in Seldarine's Hand right now, so I'll be getting the Wizard Armour and +5Dex slippers.

Will he become fatigued after casting Haste on him? Which means my Cleric will have to remember an equal number of Exhaltations?

My Cleric is also my Druid, so there is no way she's gonna be casting any Barkskins. Any Cleric spells to hike AC?

Haste no longer causes fatigue, so no worries there. In fact, you might want to give the Golden Heart of <CHARNAME> to your high AC character as this will give a permanent haste effect, thus reducing the number of spells you need to keep active on him. The other abilities of the Golden Heart are just so sweet, a must have once you enter into HOF.

There are some very good clerical spells that increase AC (Divine Shell gives a +6 deflection bonus, the highest in the game and not obtainable in any other way). The problem is, the really good spells, like Divine Shell, are self-cast only.