Well, you only said "hardness 20" before, so you are going to get a bunch of generic answers.
What printed material is this? Is the encounter meant to be a Kobayashi Maru test (you are intended to fail)? What's it's listed CR?
CR 11 (epic for APL 8) would have defensive stats of 145hp, good save of 14 and AC 25.
CR 11 is a
Stone Golem.
If it's tougher than that, I'd call foul on the writeup. If it has very low hitpoints and saves, and perhaps low damage on each of those three attacks (and no other abilities on top of the grab), then it might be fine.
Slam it as hard as you can, with a Freedom of Movement up and maybe even Shield Other to soak damage. Aid Another to make sure you damage dealer can hit and go to town.
Without more information, I'm afraid I can't say what would work or not, though.
*Edit*
Oh wait, is this something that was made with Animated Object?
If this is the case, then I'd question the validity of the build (as an appropriate encounter for APL8):
- It only starts with one slam attack, and you can buy another one with extra CP, but it's questionable if you can keep buying more above that (why not just give it 10 attacks then?).
- The spell is 6th level, which means 11th level caster created it (Epic level for APL 8 already). This allows a Huge object (1 small creature per level, 8 for Huge, 16 for Gargantuan, so Huge is the limit).
- Huge starts with 4 Construction Points (CP), at CR 7. Adamantine metal requires 6 alone. This brings the CR up to 8 (+1 CR per 2 extra CP), before any other modifiers. Adding Grab and another Slam attack brings this up to CR 9. Start tacking on more stuff and we are looking at CR 10 or higher.
If this is by spell (even with permanency), then a Dispel Magic (or anti-magic), is entirely viable. If this was made with Craft Construct (to prevent dispelling), we are looking at a 35-40kgp creation (~20k cost). This is closer to 13th level+ in NPC wealth (if you consider spending ALL his cash on this one thing alone).
Regardless, I'm not sure why Adamantine weapons aren't an option here either. Seems like things are stacked against the party.