I was making a monster for some of my PC's to fight in a semi large battle coming up in a session or two. this monster's only goal is to stand there and provide flanking bonus to another enemy. currently i have it with 407 HP with DR 5/--. Its only attack is a slam that has +16 to hit and 1d8+6 damage. I am terrible at judging CR's for things and I think that its about a CR 9. I was just wondering if i could get an opinion on this from anyone so that I might scale it a bit to fit the needs of the encounter. Thank you very much.