To create immortal thing remove all [BREATHE][GUTS][CIRCULATION][UPPERBODY][LOWERBODY][STANCE][NERVOUS][THOUGHT] from bodyparts, check all connections and replace such tokens [CONTYPE:LOWERBODY] with something like this[CON:LB]
Then add to creature file some intersting tokens:
[NOBREATHE][NOTHOUGHT][NONAUSEA][NOPAIN][NOBLEED][NOEXERT][NOSTUN]
and maybe this:[NO_SLEEP][NO_DRINK][NOFEAR]
and some immunities:[FIREIMMUNE][FIREIMMUNE_SUPER][PARALYZEIMMUNE]
and make sure it heatproof: [HEATDAM_POINT:40000][IGNITE_POINT:40000][COLDDAM_POINT:0][HOMEOTHERM:10067][FIXED_TEMP:10067](add after [STANDARD_FLESH])
if i remember right, adding megabeast and power tags prevents atom-smashing
It also can be upgraded by [MAXAGE:20000:40000]
and this thing should know how to swim [SWIMS_INNATE][SWIM_SPEED:500][AMPHIBIOUS](and maybe fly)
High size, damblock, and natural armor also help to prevent destruction of hands
Edit:[trAPAVOID][BUILDINGDESTROYER:2][WEBIMMUNE] to protect from traps
and [POPULATION_NUMBER:50:300] [CLUSTER_NUMBER:10:15][FEATURE_ATTACK_GROUP] for more fun(not sure if any civilization can survive worldgen if there 300 megabeasts)here example of such thing(it not so powerfull but hard to kill)
[CREATURE:LIQUIDHORROR]
[NAME:liquid horror:liquid horrors:liquid horror]
[TILE:1][COLOR:0:3:0]
[PREFSTRING:liquid body]
[BODY:LIQ]
[ATTACK:MAIN:BYTYPE:GRASP:strike:striked:3:7:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTYPE:GRASP:poison:poisoned:1:2:GORE][ATTACKFLAG_WITH][SPECIALATTACK_INJECT_EXTRACT:100:100]
[ATTACK:MAIN:BYTYPE:GRASP:attack:attacked:0:8:GORE][ATTACKFLAG_WITH][ATTACKFLAG_CANLATCH]
[NOSKIN][NOMEAT][NOSMELLYROT][NOSTUCKINS][SEVERONBREAKS]
[NOBREATHE][NOEXERT][NOFEAR][NONAUSEA][NOPAIN][PARALYZEIMMUNE][NOTHOUGHT][FIREIMMUNE][NOBLEED]
[NOCTURNAL][EVIL]
[DAMBLOCK:5][DIFFICULTY:5][SIZE:15][GENPOWER:3][PET_EXOTIC][trAINABLE][PETVALUE:1000]
[MODVALUE:10]
[BLOODTYPE:0]
[liKES_FIGHTING][MAGMA_VISION][EXTRAVISION]
[CAN_LEARN][CAN_SPEAK][CANOPENDOORS][trANCES][EQUIPS]
[LARGE_ROAMING][CARNIVORE][LARGE_PREDATOR]
[FAT:10][SPEED:750]
[STANDARD_FLESH]
[EXTRACT:liquid poison:10:0:1]
[EXTRACT_STUN_INTERMITTENT]
[EXTRACT_SIZE:25]
[LAYERING:100]
[BIOME:SUBTERRANEAN_WATER][BIOME:SUBTERRANEAN_CHASM][AMPHIBIOUS]
[UNDERSWIM][SWIMS_INNATE][SWIM_SPEED:500]
[STANDARD_FLESH]
[HOMEOTHERM:10040]
[MAXAGE:1000:50000]
[CHILD:100]
[liTTERSIZE:1:2]
[CLUSTER_NUMBER:4:10][POPULATION_NUMBER:10:50][FEATURE_ATTACK_GROUP]
[FREQUENCY:100]
[SPHERE:DARKNESS]
[PERSONALITY:ANGER:60:75:100]
[BODY:LIQ]
[BP:1P:mpart of body][SMELL][SIGHT]
[BP:2P:1part of body][CON:1P][SMELL][liMB][SIGHT]
[BP:2R:1rpart of body][CON:1P]
[SMELL][liMB][HEAR][GRASP]
[BP:2L:1lpart of body][CON:1P]
[SMELL][liMB][HEAR][GRASP]
[BP:3P:2part of body][CON:2P][SMELL][liMB][SIGHT]
[BP:3B:2rpart of body][CON:2P]
[SMELL][liMB][HEAR][GRASP]
[BP:3L:2lpart of body][CON:2P]
[SMELL][liMB][HEAR][GRASP]
[BP:4P:3part of body][CON:3P][SMELL][liMB][SIGHT]
[BP:4B:3rpart of body][CON:3P]
[SMELL][liMB][HEAR][GRASP]
[BP:4L:3lpart of body][CON:3P]
[SMELL][liMB][HEAR][GRASP]
[BP:EL:epart of body][CON:1P][EMBEDDED]
[BP:IL:ipart of body][CON:1P][INTERNAL][liMB]
[BP:SL:spart of body][CON:1P][SMALL]
[BP:GL:gpart of body][CON:1P][liMB]