Cool, I was thinking it was 4ish, but that is mostly due to the DR and HP. (the HP matter since this was creature 'made up', his HP are more typical of 5HD.) But in hindsight, it wasn't doing much damage either, so they were not in too much danger.
I will probably go with 3, since they were hard pressed to damage it. (I know, CR technically doesn't deal with 'difficulty', but that can be added on as a function of the encounter. I am just moving that step up a bit.)