I initially guessed 50% health was considered "low health." If you were to melee attack light enemies (i.e., non-heavy enemies like most Bullymongs) at 50%, it would cause them to stagger. Though after experimenting, this doesn't seem to be the case.
Based on my observations, I'd say "low health" is ~33% health (or 1/3 health).
Fortunately, it is pretty clear when the ability was applied in a kill. Normally you deal a fixed amount of damage to everyone. When the ability is applied, you see a boost in the damage dealt and you get a nice "buzz" sound and a slashing animation for every hit.
To test this, I respec'ed Zero and went off to fight making sure I didn't level up any more during the tests. As a baseline, I fought off enemies with only the Deception skill and no other skills. Every melee attack I dealt gave 55 damage.
I saved and restarted to go on another run. This time I added Killing Blow Level 1 to the mix. Again, most hits dealt 55 damage. Using a weak gun, I chipped enemies' health to around 50% (as close as I can get it) and hit them with a melee attacks. Still did 55 damage. I then tried for 25%. Again, as best as I can, I chipped them down to as close as I can to 25% then hit them with a melee attack. I got boosted damage and other indicators of a successful hit. So it's definitely applied at least at 25%.
However I noticed that at times when I wasn't quite at 25% but a little higher, I was still able to get the successful hit. So apparently it was higher. After a few more tests, I came to the conclusion that it is at 33% at max.
To see if the level of the skill affects this, I tried again with Killing Blow Level 2 and up. You get even more damage of course but it still seemed to apply at 33% and didn't change.
Doing some research as I was wondering this myself. . .
This forum post at Gamefaqs suggests that:
- (Most?) legendaries can drop anywhere/from anything but
- The chances in most places are extremely low and
- Certain bosses/mobs have significantly higher chances to spawn certain legendaries. Does this apply to certain chests as well? I don't know.
- It seems some legendaries only drop from certain bosses (e.g. conference call). Meant to come back and add this but forgot!
I am doing further research.
Best Answer
In general, the level of enemies you face is only determined by the following:
Everything else doesn't affect enemy level. So if you're a level 31 character in a level 17 area, what you want to do is to waltz your way to playthrough 2, jumping through the required hoops.
Good news is, when you're hosting your game and you reach the starting point of a mission you have already completed in another session, you get one (and only one) chance to play through that mission again; if you decline or restart the game, you will be credited for the quest progress you have made in your friend's game.
What you need to do is play enough of the story up to the point where you can skip all the parts of the game you've already completed with your friend. This will bring you to a more level appropriate quest selection.