dtgreene: I believe that enemy resistance is purely percentage based, in that the damage dealt by the attack is reduced by a certain percentage. Note that Physical is a damage type, and enemies can have resistance, or even immunity, to Physical damage, in which case normal attacks will not work at all unless you're using a weapon that deals added elemental damage. There is no weakness mechanic; an enemy will never take more than full damage from your attacks.

There's one very special weapon in Clouds of Xeen that ignores resistances entirely. Said weapon is not as powerful as obsidian weapons, but in some situations it actually does more damage. (This weapon is mandatory to beat Clouds, so don't worry about missing it.)

Another mechanic is enemy saving throws: An enemy that is hit with a spell (but not a weapon attack) has a chance to save against it; most damage spells will do half damage if this happens. (Mass Distortion, which seems to be capable of working on any enemy that isn't immune to all elements, is an exception; it deals 5 damage when the enemy saves instead of half the enemy's health.) The enemy's chance of saving is based off its index number; enemies with lower indexes in the array of enemy types are less likely to save. In Clouds, the stronger the enemy, the higher its index number, so stronger enemies are more likely to resist your spells; note that this is not true in Darkside (allowing spells like Dragon Sleep to be useful).

One way to get an indication of how much damage your attacks are doing is to cast Identify Monster; if the enemy has less than 1,000 HP, the spell will reveal the HP the enemy has remaining.

As far as dungeon traps, you can't avoid them in the general case, but you may be at least able to mitigate them. Specifically:
* If you have non-zero resistance to the element, you have a chance to save for half damage; if successful, you will keep getting more chances until you fail, with each save halving the damage you take. (This is how a certain chest that deals 10,000 (IIRC) fire damage can be survived.) You need at least one point of resistance, but if you have it, having high luck will give you an extra bonus. Unlike enemies, you don't get Physical resistance. (AC can keep enemy physicals from hitting, but it is of no help against traps.)
* Protection from Elements will reduce the damage by a certain amount. Note that Protection from Elements provides more protection than Day of Protection (assuming your level is less than 200), and that the spell you cast last is the one that counts. So, if you keep being hit by poison, just cast Protection from Elements to protect your party from it. You can't defend against Physical, Magic, or Energy damage this way.
* Power Shield (included in Day of Sorcery at full strength) will reduce the damage taken from all sources, including things like falling damage. It's quite helpful, especially given the next point.
* Some enemies can cause status ailments with their attacks. If you can reduce the damage to 0, the status ailment from the attack will also be prevented. Hence, if you're fighting enemies that don't do that much damage, but can hit you with nasty status ailments, you might want to cast Power Shield and maybe Protection from the Element that the enemy's attack does.

Note that what I've posted here also applies to Isles of Terra, except that there's no special weapon that ignores resistances. It also applies to Swords of Xeen, which does have resistance ignoring weapons.

Pocketim: One other thing I was wondering about unrelatedly are the traps, specifically the type where standing on a certain tile has you get hit by some gas or flames shooting out of the ground, like the poison gas in the dwarf mines and the curse gas in the sphinx. Is there any way to disarm them if you know where they are? It's annoying any time I'm backtracking through a linear hall and there's seemingly unavoidable damage and status effects that I just have to deal with.

Waltorious: I believe (not certain) that you can use the Jump spell if you have it (or Teleport spell) to avoid the trapped spaces. This only works if there's an open space just past the trap to enter, though, so it the trap is in a corner then you'll have to trigger it.