I notice that the zombies are minding their own business in their graves until they smell food which lures them out into the world to die even though they posed no visible threat in the first place.
I can't seem to finish the race mission because the airspace is so full of blimps and helicopters, I know they add a little extra challenge to the game, but really? there's got to be a wall of blimps?
The guards are all wearing power armor and even if you could somehow get the knife past the armor and kill one, the others would see his vitals go flat on their HUD and be alerted, thus losing you the Ultimate Achievement so it will never be profitable to kill the guards.
 "Q - Compressing the Heart"){kind=icon}
 "The Visitor: Massacre at Camp Happy"){kind=icon}
 "Orton and The Princess"){kind=icon}
 "Icycle"){kind=icon}
 "Warfare: 1917"){kind=icon}
 "The Last Stand: Dead Zone"){kind=icon}
 "Coinbox Hero"){kind=icon}
 "SkullFace"){kind=icon}
 "Lucky Tower 2"){kind=icon}
 "Snail Bob 3"){kind=icon}
 "Nuclear Plant"){kind=icon}
 "Transmorpher"){kind=icon}
 "All We need is Brain"){kind=icon}
 "Design your own zombie game"){kind=icon}
 "Slender - Web Edition"){kind=icon}
 "Into Space 2"){kind=icon}
 "Death vs Monstars 2"){kind=icon}
 "Ultimate Assassin 3"){kind=icon}