To expand on my previous idea, consider that although the four dimensional creature shares three dimensions with the PCs, they don't all have to be dimensions that the creature itself is aware of. What I mean is, considering that the creature can reach through an extra dimension past the PCs' armor and flesh to attack them from the inside out, perhaps one of the creature's dimension is likewise vulnerable to the PCs. Imagine that as the rounds pass, the PCs notice that between its defensive and offensive positions the creature unknowingly has its vulnerable parts similarly exposed for a few second. Perhaps when the two horned spheres split apart into twelve, some of the spheres are soft, fleshy and pulsing. If the PCs ready an action they could target these fleshy parts and so attack the creature from the inside out as well even if they can't tell that's what they're doing.That had occured to me too, though it would be a little more one sided. If these flat creatures, lets say they look like flat star shapes on the ground, attacked the PCs they could only target those parts of the PCs that they could see, in this case the soles of the PC's boots that are on the ground with them. To become completely invulnerable to the 2D creatures the PCs would simply have to fly to stay above them and effectively invisible. The PCs could then attack the 2D creatures' soft insides from above. On the plus side though, the 2D creatures would be immune to bludgeoning attacks since, being flat, they can't be crushed any thinner. It might make an interesting role-playing encounter but 3D creatures have too many advantages over 2D creatures unless you attribute them some special attack form.
