Take a read of the relevant section on Page 13 of the Rule Book. It describes the definition of 'Completely Destroyed'.
Simply destroying an enemy unit during the required turn/phase regardless of whether some the unit's models have been removed as casualties or have wounds.
This is the correct interpretation because nowhere does any objective that requires "...an enemy unit [to be] completely destroyed..." also impose the condition that the target unit must be at 'full strength' (i.e. not suffered any wounds and/or casualties).