As you progress through the game you will encounter four different Romanov models (D, F, G and N), but the model has no bearing on the achievement.
Where your first encounter takes place varies depending on the difficulty level with the first possible encounter being Act 1-3: Giant. However if you've either not encountered one there or missed that opportunity then load
Act 1-7: Seismic as this checkpoint is located immediately before a fight which includes three Romanov's regardless of difficulty. You'll need to damage the Romanov prior to your melee attack as even on Casual a single melee attack doesn't do enough damage to destroy one outright.
Whilst you can attempt the two other Romanov dependent achievements (
Brutality Bonus and
The Hand of God) at the same time this isn't the ideal location for those due the presence of an allied railgun.
These are the Romanov models for reference
http://imgur.com/a/CrkTc