Kodos, I've done a small mod that adds a controling crystal to the head of the colossus. Once you shatter the crystal, the colossus die. It depends a bit too much on luck (since you can't aim where your attacks go), but it works perfectly.
Add this to the "body_default" file in the raw/objects folder:
[BODY:CRYSTAL_CONTROLLER]
[BP:BRAIN:controlling crystal:STP][CONTYPE:HEAD][THOUGHT][CATEGORY:CRYSTAL]
[DEFAULT_RELSIZE:200]
And replace the bronze colossus entry in the "creature_standard" file with this one:
[CREATURE:COLOSSUS_BRONZE]
[DESCRIPTION:A gigantic magic statue made of bronze and bent on mayhem.]
[NAME:bronze colossus:bronze colossuses:bronze colossus]
[CASTE_NAME:bronze colossus:bronze colossuses:bronze colossus]
[CREATURE_TILE:'C'][COLOR:6:0:0]
[MEGABEAST]
[ATTACK_TRIGGER:80:10000:100000]
[FANCIFUL]
[POPULATION_NUMBER:15:30]
[NOPAIN][EXTRAVISION][NOBREATHE][NOSTUN][NONAUSEA][NOEMOTION]
[NOEXERT]
[NO_DIZZINESS]
[NO_FEVERS]
[BUILDINGDESTROYER:2]
[LARGE_PREDATOR]
[NO_DRINK][NO_EAT][NO_SLEEP]
[SPHERE:METALS]
[SPHERE:STRENGTH]
[SPHERE:WAR]
[CANNOT_UNDEAD]
[CANOPENDOORS]
[NOT_BUTCHERABLE]
[BIOME:ANY_LAND]
[EQUIPS]
[NOFEAR]
[PREFSTRING:height]
[NOBONES]
[BODY:HUMANOID:2EYES:2EARS:NOSE:HUMANOID_JOINTS:5FINGERS:5TOES:CRYSTAL_CONTROLLER]
[TISSUE:BRONZE]
[TISSUE_NAME:bronze:bronze]
[TISSUE_MATERIAL:INORGANIC:BRONZE]
[MUSCULAR]
[FUNCTIONAL]
[STRUCTURAL]
[RELATIVE_THICKNESS:1]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[TISSUE_LAYER:BY_CATEGORY:ALL:BRONZE]
[TISSUE:CRYSTAL_ROCK]
[TISSUE_NAME:crystal rock:crystal rock]
[TISSUE_MATERIAL:INORGANIC:CRYSTAL_ROCK]
[MUSCULAR]
[FUNCTIONAL]
[STRUCTURAL]
[RELATIVE_THICKNESS:1]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[TISSUE_LAYER:BY_CATEGORY:CRYSTAL:CRYSTAL_ROCK]
[BODY_SIZE:0:0:20000000]
[BODY_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[ATTACK:PUNCH:BODYPART:BY_TYPE:GRASP]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:punch:punches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[ITEMCORPSE:STATUE:NO_SUBTYPE:INORGANIC:BRONZE]
[ITEMCORPSE_QUALITY:5]
[DIURNAL]
[SWIMS_LEARNED][SWIM_SPEED:2500]
The crystal will be made of crystal rock, but you can make it any material you like by changing TISSUE_MATERIAL token.
Also, if you think it's too cheap to make it an external target, just add the INTERNAL token to the body part, so you'll have to penetrate the head to shatter the crystal.