In Orberg's Exercitia Latina Cap XLI : the following is asked:
Quid Hercules fecit Caco?
I answered: Hercules Cacum clava necavit.
Orberg's answer is: Hercules Cacum clava percussum necavit.
'percussum' seems to be a perf passiv participle to agree with Cacum: Hercules Cacum beaten with a club killed him.