Forget solo monsters. Nothing in the monsters manual, except perhaps the Tarrasque, can go toe to toe with a party with Sharpshooter Fighters/SorLock/etc in it.
How exactly are these high level sharpshooter fighters of yours making DC 17 to 21 Wisdom saves against Frightful Presence of Dragons or Dracoliches?
How exactly are these high level sharpshooter fighters preventing (for example) an ancient Black Dragon from grappling them, and then in 1.5 rounds of flying straight up 200 feet (i.e. 80 foot round one normal move, 40 foot wing attack round 1, 80 foot round 2) and attacking and then dropping them? That's a potential wing (15), tail (17), bite (19), claw (15), claw (15), frightful presence, drop (70) = 151 potential points of damage (less if any of these attacks miss, more if they critical). This is without a single spell (Haste could help out here a lot). In these two rounds, the rest of the party is not going to be taking out the dragon. Granted, a PC spell caster MIGHT have feather fall, but mostly, a dragon grappled fighter is in some serious trouble. With Haste, the Dragon could throw a breath weapon on top of this. A fighter might be able to escape the grapple, but he's still going to probably take tail and wing damage (32) and fall 120 feet (unless his turn is immediately after the Dragon's in which case he'll only fall 80 feet). That's still 74 likely points of damage in a single round.
The main chance the fighter has here is to avoid the grapple in the first place.
In fact, what stops the dragon from flying away from the PCs with the Fighter, pummeling the Fighter as it flies away and climbing higher and higher into the sky? Why would an intelligent solo dragon try to duke it out with PCs?
And, this also applies to lower level adult Dragons of size Huge or greater. They can move at full speed with a grappled PC of size medium or smaller.
In fact, lower level adult Dragons with legendary actions are even more deadly with this tactic. The 20D6 200 foot fall hurts a lower level fighter a lot more than it does a higher level fighter.
But, here's the real issue. Most intelligent powerful monsters would not be solos. They would have powerful minions, traps, lairs, and a wide variety of defenses. Even Dragons have to protect themselves against other Dragons. So yes, there could be some solo Beholders or Medusa's or some such, but most creatures should be found in groups for protection and power.