On the "The Purification" quest for the Dark Brotherhood, use the following trick to kill some of the members in the sanctuary easily, such as Ocheeva and Vincent. First, get your Disposition with them as high as possible, then sneak attack them with a dagger. Before they have a time to retaliate, propose a yield. You can repeatedly attack and yield until they are killed.
To easily kill Vicente, get plenty of garlic. Pick pocket him, and shove all the garlic into his pocket. This will work best if you have high magic, preferably with fire spells.