Overall Strategy
The bad guys come at you in waves in IWD - more often than in BG2. With HoW adding a 'call to arms' you won't be able to sneak around just out of the fog of war, picking off enemies. What does this mean?

Thieves/Rangers will still be useful scouts but backstabs will be relegated to taking out lone sentries and powerful individuals - you won't be able to backstab your way through the game without being swamped by the cavalry showing up. They have changed backstab in accordance - you can now sneak attack as well, which is easier to hit but less powerful.

Mage heavy parties may be hard pressed. Practice melee & defensive spells - maybe charm types to turn the enemies against each other. Make their number work for you (remember - summoned creatures are limited in number).

Clerics will be useful (defensive harmony, bless etc.). Looks like there's lots of high level undead too… Druids have good offensive-defensive spells, entangle for example. Unless they make Druids more powerful they will be seriously weakened by the class level restriction.

In general, you will have to rely on fighters - so base your tactics on this. Turn your mages into fighters. Clerics make good fighters too. So what to do in the battle? When hordes of enemies are crashing down on you, form a defensive line or cordon protecting your spellcasters. Don't let the hordes through and cast defensive spells on the party and offensive spells just out of range of the fight - catching front line bad guys and any reinforcements coming along. With paladins and rangers the 2nd level spell 'protection from fire/cold' is a godsend that your regular cleric doesn't need to bother with. Memorise and use to protect not necessarily from the enemy - then you can toss fireballs into the frontline with impunity.

Submitted By: Gruntboy