Velvet Assassin

enemies total: 25

sewer : 1
first shacks: 3
sidestreet: 2

checkpoint

courtyard: 4
inner yard with shacks: 4
third shack part: 1

checkpoint

littered street: 1 sniper (window at the end of street) + 2 guards
bridge and ruin: 2 snipers (2nd and 3rd level of bombed house)

checkpoint

prison yard: 3 + 2*

*the last two guards only appear when you alert one of the 3 soldiers in the yard. they have to spot you. you may flee outside again.

{Still listed as a secret achievement at the time of writing}

Miss Merciless - Unlocked on the Third Man - Ghetto level (8)
Basically for this secret achievement (and secret objective too) you have to kill EVERY soldier on this level. There are 25 and the 2 you might miss are at the very last section of the level the bit after you crawl through a gap in the wall. You need to alert, at any point, one of the 3 guards on the street near the tanks, this is the only way to get the final 2 guards to 'appear' the first time. Then just kill them all.

I did it simply by crawling through the gap, sniping the general at the end of the street, then ducking back into the hole, waited until they'd all calmed down, then repeated it, taking down a different guard. The achievement pops during her little speech about not being able to kill all of the baddies in the world but at least she had cleaned up here.

Or something :)