I hardly know what to say. No wall of force?
One didn't seem useful in this adventure. The wizard in question normally prepares one...despite the fact that I don't like it as a spell. It has come in handy more than once.
Maybe I'm missing something, but why didn't the party just run straight through?
They were faster than us, they could use life sense to track us, the darkness made it difficult to move quickly since the path was not in a straight line and we had to avoid running quickly to avoid running into the bushes. They were not limited by any of that, given their life sense, flying, and incorporealness. Running wouldn't have worked. Plus, it was a Living Greyhawk adventure. If you don't fight the encounter, you don't get the XP for it. If we weren't going to get the XP for it, we might as well have not played.
I would hope the DM wouldn't say something like that, because it contradicts the rules. You can't move before and after an attack (unless you have spring Attack) and by definition the attack occured or you wouldn't have used your readied action. It is no different than making an AoO or using a counterspell. Would the DM say your counterspell is ineffective because the spell "hasn't happened yet?"
Let's just say that we all agreed that although the exact text of the rules said you could ready a move or a wall of force to block someone's attack we pretty much all agreed that it was likely a bug in the rules and not intended due to the fact that a wall spell or a ready to move suddenly became a MUCH better option than most other options in the game. There are a number of spells that add something to people's AC for one round that become kind of useless if you can throw a wall in the way of someone's attacks AS they are attacking. Which seems counter to the balance the game creates. We all agreed to use the letter AND the intention behind the rules.
Plus, one of our DMs(since we played mostly Living Greyhawk and had 10 or so DMs) said "If you want to accept that readying to move is a valid method of avoiding attacks, that's fine. But the enemies will know that. This means that all melee characters will never hit with a single attack...ever. And as much fun as an entire encounter being: You attack, he moves out of the way is, let's just agree not to do that. Ever. Either that or I'm surrounding you with level 1 warriors who aren't worth XP who will get in your way all the while readying to get out of the way of your attacks while the high level archers kill you."
At party level +3, yes, it's going to be a hard fight, but I don't think you've made your case that the cleric saved you. With another wizard in the party instead of a a cleric, maybe you would have had some more magic missiles and a wall of force... if you ready a wall of force, you can use it to block just about any attack, from dragon breath to a spectre's touch. Maybe if there were another barbarian in the party, he might have had a ghost touch weapon as a backup.
See above for "using Wall of Force to block attacks". In our game, it was a spell that you could put across a room to prevent the Wizard in the back from affecting the combat or to put across a door to prevent the enemies from running away. And when I say "In our game", I mean, I never say someone in Living Greyhawk do that ever. Even when I played with high level Wizards at GenCon, as well as other conventions all over the world. In fact, the idea that it could be used to block attacks as a side effect of the above rules loophole didn't occur to be until you said it in your post. Even if it was allowed, it would require a new Wall of Force during each round of the combat as they simply would stay in the ground, pop out of the ground on the other side of the wall and attack. Then spring attack back in again.
Magic items were also in short supply in Living Greyhawk. The idea that someone had an "extra" magic item to use is...kind of laughable. My level 15 Cleric who was just about retired had a +2 Falchion as his primary weapon. His backup was a non-magical Adamantine Heavy Mace. He didn't own a ranged weapon. This didn't much matter because he had ranged spells and the ability to cast Greater Magic Weapon. But if I ever got enough money to afford a +1 Ghost Touch weapon...I would spend it on something else. Ghost Touch was of so limited utility that it wasn't worth spending money on it when you could have spent it on a better Ring of Protection or as a payment towards a better Wisdom enhancing item.
Magic Missiles help, I admit. But having enough of them to deal with a Dread Wraith is unlikely. Too many hitpoints to go through. Either way, the Wizard would be down to nothing BUT Magic Missiles in no time. That's the problem with losing a spell for every negative level you get. And these were intelligent undead. I would drain him dry in 2 rounds, assuming 3 of the 4 Spectres hit each attack.
As for the ease of the encounter. Our party was extremely optimized. We were used to blowing through EL=APL+3 encounters on a regular basis. It was only the APL+4 encounters that even phased us. But EL is a really, really poor estimate of difficulty, so that doesn't say anything. One APL+1 encounter could be anywhere from impossible to extremely easy depending on the party makeup, the spells prepared, and the magic items they had. Which is kind of the point of this discussion.
I have rarely seen a cleric take a special role against undead in my games, except for the occasional death ward. Empowered magic missile is a popular spell.
Well, against most undead...no, I'm no more effective than anyone else in the party when I was playing my Cleric. I was poor at turning. But against incorporeal undead there were only 3 real ways to get past the miss chance: force spells, Ghost Touch weapons, and Healing spells. A well placed Heal spell on a Dread Wraith spelled its death. Too bad I never got one off during this encounter due to the tendency for the Dread Wraths to attack from 2 squares away.