Best level to do this is obviously the first stage, as the majority of encounters will be before you actually get Amy, though there is the bottleneck at the end of the chapter with the five or six zombies released from the cages. Now there are two ways you can go about this.
One is when you let Amy crawl into the room to open the door to this area, rather than letting her out leave her in there and dispatch the zombies yourself and then let her out when you've disposed of all of them. Be warned though, combat is a massive pain in the ass in this game; fighting an enemy one on one is annoying enough let alone the inevitable 3 or 4 you'll get on your tail. There is a rhythm to the combat; you should dodge and then try and sneak one or two hits in and repeat, but given how buggy the game is hits sometimes will not register, and each of them take about 3 or 4 hits to go down on Hard. The zombies also seem to have some rudimentary martial arts skills as well, as they will oftentimes dodge your attacks and go in for counters (I thought they were supposed to be brainless killing machines?). A lot of people swear by this method, but it's not the one I used and it's pretty tricky especially when you're just learning the game. Go back on a lower difficulty if you're trying this method I reckon. Also if you die during this method Amy will be with you at the trigger to it as the game intended you to release her right away, so you have maybe one attempt to do this.
The second method is to quite simply just run. As soon as you open the door, piss bolt through the small maze to the door to safety and let Marcello and Amy in when you see them as they will follow you. Amy was miles behind when I got to the safe area, was waltzing past zombies and I still got the trophy at the end of the chapter, so that indicates to me at least that they by and large ignore her for the most part; they mainly target Lana.
Whatever you choose to do, Chapter 1 is the best option.