Will PCs going to raid villains' lair? Or will villains going to raid PCs' home ground? Or will they meant to meat in some other place?
That will change the situation a lot. If PCs must raid the villain's lair, bad guys can use long-term defensive spells such as Unhallow, Forbiddance, vairous Symbol spells and/or other traps and dungeon features.
For defense, I always prefer to have enough number of soldiers in addition to BBEGs. Even how strong the villains are, if 18th-level PCs can concentrates their attacks, they will fall soon, very soon.
Regarding summoned monsters. Well, if that is a 18th-level game, allowing villain cleric to use Gate for calling (not summoning) some extraplanar critter will be acceptable. If their alignment is LE, how about some Pyroclastic dragon or Rust Dragon?
Most likely a neutral type of location that will allow both sides to do a bit of buffing first - of course, the evil cleric could Gate in a Pit Fiend or one of the dragons you mentioned. Where is the pyroclastic, by the way?