Well, I'm having some difficulty making it spellcaster-proof as much as possible. While it does have a a SR of 29 (w00t) I'm sure there are some magic items which would help it even more ... must dig up DMG and find out.
EDIT
Actually, as a construct it is immune to many things. Spells like
Wail of the Banshee, Implosion (I think ... it says creature; does that apply to objects as well?), Harm (same as implosion; says creature), Bigbys (can be destroyed easily enough), Power Word: Kill (too many HP) ...
Ones that are 'normal' spells are: Meteor Swarm, Disintegrate, Incendiary Cloud, Fire Storm, etc ... standard high powered Evo spells.
It has a lot of immunities: no mindaffects, necro, sleep, paralysis, stunning, death effects, spell w/ fort saves (unless apply to objects) and no crits.