But do you see how a CR 6 with a Offense of 9 and Defense of 1/2 could be a problem in the hands of a inexperienced DM?That would be a glass canon! And there is nothing wrong with that. You are not required to have a balanced CR. But I will check the numbers:
Defensive CR 1/2
Offensive CR 9 (at least that is what I get)
IMO, the CR should be 4.5, so round 5.
The offense of a CR 9 monster kills most 6th level PCs in 2 rounds.