Before Niko goes in for his interview at the law firm, equip him with a knife. During the interview, the instruction caption will tell you to shoot the lawyer. Doing this will make your escape harder as you will have to deal with security guards and the police if you leave through the front door (or a group of police if you jump out the window). Instead, kill the lawyer using the knife and grab the evidence. You can just walk out as if nothing happened. Note: This may also work on any mission requiring a single decisive kill on an unsuspecting target.