Adaptive forcefield. An assimilator can only be damaged by each energy type once. Once it has been damaged by a particular energy type (heat, cold, electricity, etc.) its forcefield adapts, rendering it immune to that energy type. This applies on a "per encounter" basis; the immunity is not permanent.
Assimilate. If a victim is reduced to zeo HEALTH, the assimilator can inject it with nanites which turn it permanently into an assimilator under the hive's control.
Hive mind. Assimilators operate more efficiently when in groups. Every additional assimilator beyond the first within 30' effecibely counts as flanking/crossfire , andgrants +1d6 to all attribute checks, to a maximum of +3d6.
Integrated scanner. Assimilators have scanners integrated into their circuitry.