I have the Challenge to do a Flashlight save but how should I do it of the Killer is baiting because he knows I have one?
Remember that the "Decisive Light" challenge doesn't actually require you to save your teammate - it just requires you to blind the killer while they are carrying someone. This means that you can blind them during the pickup animation, in which case you probably won't save your teammate but you will get the challenge.
So I would say hide behind a rock or something and run out and shine it in their face until the beam widens as soon as they start to pick the person up, then run away quickly. If they're facing the wrong way, or if they look around and find you before picking up your teammate, there's not a lot you can do. Just keep trying, and eventually you'll find yourself in the right place at the right time. It took me about half a dozen games to get that challenge - it's definitely one of the harder ones in the Tome, but it's absolutely doable.
Wait until they pick someone up, or commit to breaking a pallet.
But what if they'll start looking around befor they pick the Survivor up?
Use Sprint Burst to quickly position yourself for the save when they pick up the survivor.
Be a stealth god.
Experience. The only way you'll get that is by practicing.