You can easily get this while boosting weapon kills in a private game (at least 4 v 4 to count, obviously).
You have 3 people dying and 1 reviving on the opposing team. Shoot the 3 people dying 3 times (human opponents) each with your pistol THEN have someone else kill them (usually whoever is boosting weapons). When they've been revived do the same again until they all eventually bite the final bullet and the round ends.
Do this for the first 3 rounds, then at the end of round 3 when only the revivor is left, kill him for your ONE and only kill. At this point your ratio should be around 10 or higher, so sit back (don't kill anything else) and wait for the person boosting their weapons to complete rounds 4, 5 and 6.
And just to clarify no-one should be casting Tree of life.