Honestly, for your average mortal, I just have the assassin roll an opposed stealth roll, and call the mortal dead.
For a goon that might actually take a consequence (meaning 3-4 stress, and a -2 consequence, so 5-6 stress to take out), now we'll do two rolls: sneak (as a maneuver) and an actual attack roll. With a 4-5 skill and one maneuver (so 6-7 plus roll) versus someone starting at 0, the enemy is usually taken out in a hit, but a careful person probably wants one more maneuver there (aiming or such).
For anything that's going to take more than one consequence, now the player needs to get really creative with taking them out. We had a sniper hit for 28 stress before without magic, but it was a series of steps done to hunt a specific target plus a 5 fate point dump... so, still, he pulled off 18 stress without fate points. That would take out probably anything but a major NPC or a PC.
The thing to remember is that as a GM, the game goes faster if you actually DO tell your players what it will take to kill the guy (as in, this dude's just an accountant, you can kill him with a successful stealth roll), vs "letting them figure it out". As the "mystery" will make sure that they always try for 20 some hit stress on every random accountant.