Our 5-member lvl7 party (two Sorcerers, two Rangers, one Cleric) managed to beat an Adult Silver Dragon (CR16).
We just spammed CC spells and made saving throw rerolls with Silvery Barbs.
It used up all its Legendary Resistances on the first round, which allowed it to Breath Weapon and take down 2 members. But then it got CC-ed, during which the downed party members were healed so they could fight again. And after that, it just never got to take any actions again.
As long as 1 hard CC succeeds from the 3 full casters, which is extremely likely, especially with Silvery Barbs spam, the dragon can't do anything, and the rest of the party can DPS.
Not only we won, it was even a surprisingly anticlimactic fight. Action economy advantage + CC is just stupidly strong, since you can just allocate part of your action economy to completely disable the opponent.
EDIT: Since this keeps being asked: The main CC spells were Command, Psychic Lance and Entangle. Aberrant Mind Sorcerers can cast Psychic Lance a lot, thanks to Psionic Spells, Psionic Sorcery and Font of Magic features. Psychic Lance causes Incapacitated, which disables Legendary Actions.
Also, our DM did not expect us to fight the dragon, hence why it was by itself and we were fully rested. One of our members requested a sparring match, but the DM ruled, for fun, that even a breath weapon could do non-lethal damage, so both sides could go all-in as if it was a real fight. The dragon yielded when it was about to go down.