KO Streaks are built by getting consecutive kills without dying yourself. Getting to a 5 KO Streak will result in you going "On Fire" which is primarily just a cosmetic change. Once you get the 8th kill the achievement will unlock.
This is easiest done in a custom match with Easy Bots, Deathmatch game type with a kill limit of 10 or more, I recommend 25 to give you lots of room to restart the kill streak if you get unlucky. I leave all Power-ups standard and add the Stack Power-ups Modifier as that allows you to have multiple power-ups at the same time. The most important one is Homing as that makes kills super easy to get.
For an Arena I prefer small to midsize with minimal levels or obstructions. I found Nexus to be perfect for most achievements in this game. Good Luck and Happy Hunting!