ACTOR MyInvisibleProjectileForHittingMonster
{
Speed 30
Radius 4
Height 8
Damage 0
Projectile
+MTHRUSPECIES
+SEEKERMISSILE
States
{
Spawn:
  TNT1 A 0
  MISL D 20
  Stop
Death:
  TNT1 A 0 A_SeekerMissile(0, 0, SMF_LOOK, 256, 1)
  TNT1 A 10 ACS_NamedExecuteAlways("ProjectileHitsMonster",0)
  Stop
}
}


ACS:

script "ProjectileHitsMonster" (void)
{
int projectile_tid = UniqueTID();
Thing_ChangeTID(0, projectile_tid);

SetActivator(0, AAPTR_TRACER);
int mon_tid = ActivatorTID();
if (mon_tid < 1) {
  mon_tid = UniqueTID();
  Thing_ChangeTID(0, mon_tid);
}

...