Majoru Oakheart
Adventurer
What, for a 12th level party? Let's imagine the party consists of a wizard, a sorcerer, a fighter, and a monk. As soon as the party figures out what's happening, the wizard casts repulsion. The fighter and rogue ready an action to shift 5 feet as soon as they are attacked, while the sorcerer readies a magic missile to blast anything that makes it past the repulsion. On the next round, the sorcerer readies a wall of force, trapping the next opponent as it tries to spring away. The wizard blasts the opponent with magic missile while the the fighter and monk now whale on the trapped opponent using their ghost touch weapons.
I can only say what happened when the two 12th level groups I saw went through that encounter(one where I was the cleric). I can say that no one had prepared repulsion...and oddly enough I've never seen it cast at all. No one in either group prepared Wall of Force, Undeath to Death or owned Ghost Touch weapons. Besides, Wall of Force would have just made the monsters go through the ground to get around it. Replusion would have caused them to retreat into the darkness and wait the couple of rounds until they had to come back and attack us.
As for the shifting 5ft when they are attacked....well, there was very little room to move. Most of us had 3 allies, and 3 squares of brambles adjacent to us during any given round. Plus, I've never seen someone use the "ready a shift when I'm attack" action ever. It was brought up as a possible rules loophole in our group a while back. Virtually everyone in our group figured that if you could either spend your standard action on full defense in order to up your defense by a small amount or you could use it to ready to make an attack miss entirely that the "ready to shift" action wasn't really a balanced or intended use of the rules. I think someone tried it once and the DM simply said "Your shift happens before the attack, so he hasn't taken it yet. He continues moving towards you and then attacks. Now can we stop readying actions to do nothing?"
What did happen was that our Barbarian managed to lose 6 levels and 5 Con points in the surprise round of combat. With -6 to his attack rolls, he couldn't hit anything. Our Ranger couldn't get a full attack sequence off since he kept having to ready for when the enemies got within sight. With a 50% miss chance, he hurt them...but slowly. We had a Wizard, but he was using readying Magic Missiles when the enemies got within range. He didn't have enough of them prepared to actually beat them.
It ended up taking a Death Ward, a Restoration(I really wish I prepared a Greater Restoration that day) and a couple of Heal spells on the Barbarian in order to win the fight. He did the most damage on a single attack out of anyone in the party, so we kept him immune to the energy drain and kept him attacking. It was hard...very hard. Probably one of the hardest encounters I've ever had in 3.5e. And without my cleric in the group, it would have been over very quickly.