I don't see how it could be last shot. I hit a group of mobs and dealt damage but killed none. I recieved 1 kill from doing this. Maybe it is a combination of both last kill and if you dealt more than 50% damage?

Another small test.

Start End Total Kills
kills 9113 9153 40 Catacombs
kills 9153 9161 8 Steam Chamber


both of these are dealing with a team of 5 players, lvl 50 -56

in front lines of the Steam Chamber with starting hit, I recived 8 kills (out of a total of 16)
In front lines of Catacombs, I received 40 kills. (out of 84)
(interestingly enough, both seem to be around half of the total mobs..)

I was constantly firing both standard attack, and spells. I felt I was dealing a comperable amount of damage as everyone else.