I ran into issues with shapeshifters since, after the first encounter with one, the players turned into "everyone is doppleganger in disguise" mode, which makes using their trademark ability pointless. Because of the story setting, players are trying to infiltrate and stop some event in one castle, the whole thing being set in the past. Hence they do not have any motivation to keep anyone alive because "it was 2K+ years ago and from the history records no one wrote about some group which killed all the cooks in that castle one night".
That being said, using a shapeshifter disguised as some servant is problematic since PCs would just knock him unconscious because "he is a level 1 minion NPC". Since the shapeshifter is certainly NOT a lvl 1 NPC minion, there is a chance he would resist the at-will power one of the players would use to knock him out (not to mention he would have more hitpoints so he would not go unconscious), immediately making the players kill him because "a lvl 1 minion wouldn't resist my spell so it must be a shapeshifter and we will kill him".
I am not happy about this approach but I do not know how to prevent it.
My idea was to give ordinary NPCs some stats too so the players could not abuse their knowledge of mechanics. This was however not a popular solution and I had to defend why the servant (who was this time really a shapeshifter trying to roll with the party for a while) resisted the spell which should have hit him if he was lvl 1 minion.
Is there any good way how to solve this issue? I would like to use shapeshifters in disguise to enhance the story but current players' mindset makes it really troublesome if they mostly attack on sight without the interest of taking any NPC along.