You can unlock this trophy pretty much anywhere in the game where there are a lot of enemies. Just be sure to alternate between guns and hand-to-hand kills. This will obviously be easier the earlier in the game that you attempt it.
You can even reload checkpoints, as long as you remember to keep the alternating sequence going in the proper order. I found the easiest place to attempt this was in Chapter 6 - The Chateau when you reach the courtyard before entering the castle in France.
Note: The trophy will unlock right after the 10th kill in the alternating sequence.