Tip 1: Remove as many guards as you can to make life easier.
Tip 2: Because guards move when you do, you can sometimes pace back and forth to move a guard into a more useful position.
Tip 3: Focus on finishing a level first before trying to attempt one of the objectives.
Tip 4: If a spinning guard keeps giving you away, you'll need to move an uneven number of spaces to sneak up behind it.