Agreed to the DR change and single actions only.
How does this look?
Immunity to Magic (Ex): A necromantic colossus is immune to any spell or spell-like ability that allows spell resistance. In addition, certain spells and effects function differently against the creature, as noted below.
A magical attack that deals positive energy damage (such as a cure spell) damages a necromantic colossus as if it were an undead creature.
A magical attack that deals negative energy damage breaks heals 1 point of damage for each 3 points of damage the attack would otherwise deal. If the amount of healing would cause the necromantic colossus to exceed its full normal hit points, it gains any excess as temporary hit points. For example, a necromantic colossus hit by an inflict moderate wounds gains back 6 hit points if the damage total is 18 points. A necromantic colossus gets no saving throw against negative energy effects.