The best solution would be to choose "The Siege of Wondertown Land", after the scene with the king you need to go to look for Lawrence, after finding him and watching the cutscene, 4-5 Muggers will appear, shouldn't take more than 3 minutes to freeze all 15
Update: Tricka confirmed that you can re-freeze the same 2 Muggers that appear at the beginning and it will count towards the challenge, so it's confirmed this doesn't take more than 3 minutes to do
As an extra note, you have to wait until they un-freeze and shoot them again for this to count, might be obvious for some but just clearing it up