It takes a bit of work...
but these levels are easily manageable. The trick is doing three things:
1) Give them plenty of stuff to whup on that has low hp, CR, and some cool special abilities. High level PCs win the vast majority of their fights easily, that's part of the fun of playing high level. Few people like their archmages and weaponmasters getting their butt-kicked consistently. But make these fights quick, or they will get boring, so make sure that these enemies have low hit points. Now, this may sound insane, but not only will this be enjoyable, it will also make your job easier. The reason is this: if you can scare them the PCs will blow their best spells and abilities against monsters they could beat without them. Ambush encounters, hostage situations, dealings with local authorities, or anything else with a "complication" usually will cause this. Just remember to keep the CR low or you'll just compound the problem.
2) Don't let them reload between fights: This is actually the hardest part. Any group can probably heal any hit point loss within minutes, and equipment (charges on magical items, potions, arrows, etc) is not likely to be a problem unless you specifically design it in this manner, so spell recovery will be your biggest friend. Remember, anything you can do to put time limits on adventures will prevent the PCs from popping into the Mordenkainen's Magnificent Mansions and getting 8 hours of rest and one more hour of quiet study.
3) Finally, when you get to your "boss," pull no punches. And bring friends. Your PCs will teach you effective tactics, use them against them. Remember, everything with a good intelligence and a CR of 11 or above will have to consider Harm, and everything CR 17 or above will be dealing with Timestop. Having multiple minions, one who can defend your bad guy (held actions work wonders for this), and one with a high spot. Just remember to let the cleric get his harm every now and then, and not to have the Big Bad Guy see the rogue every time he comes up on him.
Hope this helps.