What I have been doing is to approach an enemy invisibly, then use a spell I made which weakens them to 100% shock for example, while firing a blast of say 25 points damage at them and summoning a storm Atronach all in one.
I did also make one which does all those three plus paralyzes them as well for a few seconds but it is a bit costly to use in terms of Magicka.
Then later, when they have been defeated by the storm Atro, not me, the people count has notched up by one, although the assault count has remained at the same level.
This has been going on for quite a while now and it seems that there is no way around it, so maybe it's not possible to complete the game without killing a certain number?