The only reason that the CR19 creature isn't that tough is because some DM's have the creature stand out in the middle of open fields, and only attack those with the most HP.
lets see - comparing him to a L19 fighter battlemaster, assuming feats and magic items
PC AC 24-26, Balor AC 19
PC has 242 hp (267 hp counting 2nd wind), Balor 262 hp
PC gets 3 attacks, Balor gets 2
PC gets +13 to hit so hits Balor 75% of the time, Balor gets +14 so hits 50% of time
PC does 14 dmg.on hit, Balor does 30 dmg.
so if they just went up and slugged it out, they would each kill each other on the 9th round on average.
PC gets action surge, a couple msc.feats, 6d12 dice for maneuvers...Balor gets teleport, fly, aura...thats alla bout a wash
so maybe it all comes down to the party composition and monster composition...a solo balor w/o healing is not gonna do so well against a balanced L7 party w/healing....just like a solo L19 fighter could be taken down by a balanced group of L7 baddies prolly