These are looking pretty close to finished.
You know, with a 20 foot reach and Dex 15 maybe Combat Reflexes would be more useful than Stand Still.
What challenge rating do you think? They look like a CR3 to me - they've got more HP than an ogre, do almost as much damage and have DR, SR and Construct immunities.
Apart from that, it's just Construction.
We should add an acid damage spell to the prereqs, acid arrow seems the best fit in the SRD.
Do we want to keep bull's strength or swap it for acid arrow?
They seem tougher than a demos magen, so I think we should increase their price. Maybe to 7,000 gp? (Cost 4,000 gp + 240 XP).