I'm not familiar with the RttToEE adventure, but there are a few ways to beat Harm.
1: It's a Touch Spell. Super high ACs will make it difficult to make that touch attack.
2: Spell Resistance. Any spells/items that grant or improve spell resistance will help out a lot.
3: Spell Immunity. This will stop Harm outright. Also very useful for stopping the ever annoying Magic Missile. It only takes someone capable of 4th level clerical magic, and I'm pretty sure there have to be at least one or two clerics in a place known as a Temple.
4) Ring of Counterspells. Another (albeit one-shot) method of stopping Harm in it's tracks.
5) Don't let the Cleric touch you. This could be tough, but using spells, reach, trip attacks, readied actions, etc., it would be possible just to simply stay away from the cleric. Other options include Wall spells, movement impeding spells like Entangle, Hold Spells... all the way down to low-tech, low-spell methods such as tanglefoot bags.
6) Mind Control. Dominate the cleric and have him Harm the fighter. I've generally found that is a good way to REALLY piss off players, especially with un-Rule 0'ed Harm.
7) Counterspelling. Any time the cleric casts Harm, have another cleric/druid (or another class using Dispel Magic) on hand to counterspell it.
8) Don't be there for the touch. Various methods exist for incorporeality, miss chances, and other things such as mirror image. This is a VERY temporary solution, because they WILL find a way to hit you. (Does popping a mirror image burn up the "charge" of Harm? I don't know.)
9) Contingency Spells. "If I am reduced below 1/4 of my hit points, Heal me." No smart dragon that can cast divine spells would go ANYWHERE without that one. Even if you are not a dragon and only have access to Arcane spells, you can turn that one into "teleport me the heck out of there."
10) Keep the Cleric from casting Harm in the first place. A grappled cleric cannot cast harm unless he had the foresight to prepare it with the Still Spell metamagic feat. A silence spell will keep the cleric from using the verbal components unless he used the Silent Spell metamagic feat. This one has another bonus -- cast it on a fighter type and have them hunt down the cleric and stay with him. Keeping the group from healing ought to slow them down pretty effectively.
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That's my top ten countdown of how to keep the Harm spell from totally dominating.
Of course, I just house-ruled the spell so that creatures get a will-save to lose only half of their current hitpoints (can't take them less than 1d4 hp, though.)