You can only use "Roulette Barrel"'s effect once per turn.
You have to roll a die twice, not once. You must roll the die twice in a row, not once in Main Phase 1 and once in Main Phase 2, etc.
You CAN select a die roll result which doesn't match the Level Stars of a face-up monster. But if the die roll you select does match a monster, you MUST destroy a face-up monster with the correct number of Level Stars.
"Roulette Barrel" can destroy itself with its effect.
You cannot destroy a monster with 7 or more stars with "Roulette Barrel"'s effect.