Savage Wombat
Hero
I read the "flesh to stone" discussion below, and it got me to wonder:
Couldn't you make a really fun encounter for your players out of a high-level wizard (or spellcasting creature) with a poor spell selection? He's much higher level than the party, but his effective CR is lowered to the party's reach by his inefficient build. Especially if you picked spells that were fun to interact with for the players, and not just ways to reduce them to 0 hp as quickly as possible.
Couldn't you make a really fun encounter for your players out of a high-level wizard (or spellcasting creature) with a poor spell selection? He's much higher level than the party, but his effective CR is lowered to the party's reach by his inefficient build. Especially if you picked spells that were fun to interact with for the players, and not just ways to reduce them to 0 hp as quickly as possible.