Lets discuss how this worked in 3E, with the level system. You have a party of 5 level 5 PCs.
Applying the CR system (+2 CR every time group doubles) 2 level 5s is a CR 7, 4 level 5s is a CR9. We'll say CR10, since we want this to be challenging, and there's 5 players.
But now we go to pick out CR10 monsters for our solo. We pick a Salamander Noble. Big critter, lots of fire damage, vulnerable to cold. Looks good.
The party druid tosses blinding spittle in its eyes. Blinded, its attacks hardly reliably hit, and it loses some AC. Then the Druid and Wizard start summoning animals around it, while the fighter and rogue rain arrows in it. With it having to kill summons (slowly, thanks to missing much more than it should) and taking summon and arrow hits, it begins to falter. The party cleric, meanwhile, is debuffing the hell out of it, making it just very ineffective, thanks to stacking debuffs.
Finally after shooting it enough, the stupid thing goes down. It failed to be a challenge in any real way for the party, because stacking debuffs and its lack of attacks meant it was mostly a punching bag.
While the party thought the encounter was a bit tedious and generally easily negated, the DM later decides that he liked it enough to try again.
This time he selects a nine-headed cryohydra...
And this is why solos require different design than 'they have a lot more levels than the PCs.'