We have a local group (near Sacramento) with 19th level characters...and the Cleric is a Rogue with a 1 level dip. :)
I believe they've played Race already, so are stuck for now.
(Rogue, Buffer/blaster Eldritch Knight, 2 Fighters, Blaster/buffer Wizard)
Edit to add: This is Chernobyl's group; he plays one of the fighters.
I ran them through Xin-Shalast and it's pretty much Extended Haste, Heroism, Stoneskin, Mind Blank, etc. and commando through with backup buffs if Dispelled/expired, DimDooring for full attacks when able via familiars with wands. Limited Wishes (or even Wishes) & UMD scrolls for the emergency recovery and Quickened Chain Lightnings (perhaps acidic) for the tough fights.
It took maybe 10 minutes of game time (once they arrived at the site).
-You do need high saves. They wouldn't have made it through without them, needing to roll in the low 20s TONS to avoid costly attrition and in the high 20s dozens of times to avoid elimination. Clear Spindles are not reliable, as they only cover enchantments/compulsions where you're given commands by an evil caster/source/spell.
-Anybody standing toe-to-toe needs an AC in the high 30s if you only want to get mauled. To stand tough, you need mid-40s because you can't let those iteratives hit you, not at 50 h.p./whack, and many monsters rely on hitting you to deliver very brutal effects.
Similar goes for CMD. Awesome Blows can add up fast, especially if the enemies readied one last round and you're prone for their full attack.
-There are no battle lines. Most everything can cover the battlefield in one move, or blast/dissolve/madden/banish at range. You should be able to as well, in all sorts of venues.
-Most everything can sense most anything hidden. You should be able to too. Many opponents can do recon on you, you should be able to too.
-Necklace of Adaptation is (nearly) a must.
-You need Quickened Dim Door, Grace, Walls of Force or Stone, Contingency-Resilent Sphere or Dim Door, etc. for those emergencies when you have less than a round to save yourself or somebody else.
((many))Improved familiars with 4th level wands can be good for quick rescues, getting full ranks for yourself to bestow ability on them if they can't use wands naturally.)
-Have at least one "deny crit" or "make enemy reroll" per day because most PCs will get a crit thrown their way that can take them out. You'll probably want your other rerolls for that '1' on a save.
-Have (Communal) Stoneskin up. It's a pittance at that level.
-Yes on the backup weapon. Telekinesis is common, as are lava pits, chasms, portals to Hell, and other hazards your weapon (or you) can be thrown into. In the same vein, boost the h.p. of your armor and best weapons. "Ignore Hardness" abilities can make short work of your gear.
-Do not rely on any one trick or a set of similar tricks. There will be something immune to it, or even that capitalizes on you trying it.
-Have energy resistance(s)/protection up, especially if you're not a tank. Anything thematic to the scenario, of course, but fire at a minimum with perhaps acid and lightning too if you're fragile.
-Pearls of Power (non-casters too!) and Metamagic Rods should be bought in bulk.
-Dimensional Anchor (or Feeblemind) can avoid many headaches, or enemies recovering and regrouping. (And works well with Force Cage too.)
-Have ranks in Fly if you actually expect to Fly in some of the conditions you'll encounter.
Much of melee revolves around who can set up their full attacks first, and which caster(s)/ranged can neutralize the other caster(s)/ranged first, and which teams can harmonize the balance of the two sides as needed. Unfortunately many of your enemies at that level can play either way, i.e. outsiders and dragons.
Lastly, being more than one round from your potential saviors can be quite hazardous.