During Mission 5: Clean House
, you will need to kill all 11 enemies, using only 11 bullets, and without being hit. It is ok to use "Load Checkpoint" when you get hit in most cases as long as you don't get an unlucky checkpoint.
This sounds a lot harder than it is. First off, use the assault rifle and not the pistol; it's a single fire gun so you shouldn't have a problem. Work your way through the house, learn the patterns, aim for the upper body, and prioritize armed enemies. A full run of this level only takes about 5 minutes.
Ground floor - 3 enemies (all in main room)
1st floor - 3 enemies (hostage taker, hostage, back room rusher)
2nd floor - 3 enemies (hugging door, tucked in right corner, in left bedroom behind extra door)
3rd floor - 1 enemy (after crying baby, they'll be in next room and get tucked under the bed before shooting)
Top floor - 1 enemy (shoot on contact)IMPORTANT NOTE:
Hostages can only be taken out after they reach for a gun and present a threat.