To get perfectionist you need to

1. Shoot every guy in the level
   
2. Make sure every shot hits a guy
   
3. Be in classic mode (no modifiers, single 15 shot gun)

It doesn't matter what your accuracy is or if you are on beat.

There's a hidden guy on every level in Classic and the two campaign sets, try it on one of the Reloaded or Heartbreaker scenes instead and you'll get it no problem.