Mistwell
Crusty Old Meatwad
I would do 12d6, relfex save DC 15 for half damage (for getting a more critical part of your body out of the way in time). Your character essentially set a trap for the golem, and a reflex save to avoid some of the trap seems appropriate (though not an attack roll...as the golem set the trap off already when he entered the square with the gate, causing the magic using member of the party to cast the spell to complete the trap).