When you're in a team, not adventuring solo! A challenge of the party level is suposed to drain 25% of their resources, a challenge four levels higher is a pretty tough but still doable fight, but both those assume the party has 4 players, not just 1.
On the other hand if the rogue player was the one saying he can solo the solar, well, good luck. He'll need it. But it still isn't level-apropriate in any sense of the word.
His claim, not mine. I would have chosen a CR 20 since that gives two different casters that would easily kill the Rogue and his low saves, 6 different dragons that all have more and better ways of detecting him and one big thing that is just better than him. He went for the Solar.
What this thread and the ones leading up to it showed is that it didn't much matter, as it was incredibly simple for the Solar or any other level appropriate enemy to beat the Rogue. If you assume a party is there, perhaps the party beats it anyways but the encounter still stopped the Rogue cold and dead.
That's why he's having to rewrite his character once again.