How about gricks. CR 3? Sure they've got DR 10/magic and a good number of attacks...but they've only got 9 hp and their attacks are fairly low damage.
DR x/magic might not even be a roadblock if the party cleric has the War domain and casts Magic Weapon on the weapon of the party bruiser.
Would CR 2 be more realistic?
Possibly. Though even cr2 seems a tad too high, but it is definitely too tough for a cr1.
I think that in extreme cases like this, you may want to redesign the grick rather than try to find a compromise for it. As mentioned, it has low hp which is supposedly compensated by decent dr (except that scorching ray does it in, a raging barb could plausibly deal that much damage on a hit), crap damage but a lot of attacks (4 tentacles can really add up).
For me, I would advance it all the way to 6HD (improving its cr to 4), then slap on the dungeonbred template to reduce its size back to medium. This improves its hp to 45 (I place the extra stat point for its 4th HD in con), attack to +8 (4 tentacles at 1d6+4 each), fort/reflex/will weigh in at +5/+4/+7.
Featwise, dungeonbred grants the endurance feat for free, which paves the way for diehard (giving that little bit of extra hp). Other possible feats include travel devotion (if you want to give it a version of pounce so it can attack with all its tentacles more consistently) or perhaps simply weapon focus.
I know that being large should technically add +1cr, but since it is arguably too weak for a cr3 to begin with, plus staying medium robs it of reach, I decided to assume it has already been factored in. Seems fair for a cr4?
