@ robertliguori: Regarding the Lich, won't that only hold him for about 8 hours? He can recover spells after then, so unless you're talking about a Permanence-ed Dimensionally Anchored, Magic Circle Against Evil-ed, Magic Circle Against Undead-ed, etc. prison, he might manage to get out. I'm not suggesting that you haven't thought it through, rather I'm curious. I chose the spells that I thought would prevent teleportation/planeswalking and domination/suggestion, but there's always the instance of finding that one spell that doesn't have verbal, somatic, material or focus requirements and exploiting it. What would you do to keep him in there?
@ topic: I think the horse is looking a bit less attractive now that it's all dead and corpsified, but I'll aim another kick at it for the hell of it. As has been said, there's a level-gap that seems to be unaccounted-for so far. Maruts are fine (@ javcs), as are anit-undead classes/items. But the biggest anit-undead measures I can think of is to stick them on the Plane of Positive Energy for a good, hard fight. The other two will manage okay (if the druid has healing spells prepared) once they deal with the 'no air' problem, but the two undead should be ill-at-ease, to say the least. I know that the Positive Energy Plane doesn't kill them automatically, but if some force were to cause a flare-up of energy bursts (3d10 temp hp gained, but let's do the math and subtract it from undead, shall we?) it would threaten the PCs equally while putting the undead at a disadvantage when dealing with any encounters you wish to stage.