Spree ender is a kill bonus that you get when you kill someone who is on a killing spree. (5 kills without dying) One of the most common campaign levels to get the kill bonus is Level 15 Alien campaign. However, if you want to get it quick fast, then follow these steps below in a quickmatch.
Quickmatch - getting it[]
Set only one opponent, medium, with at least 10 points to win.
Explosives, and arm yourself with a rocket launcher
Equipment off
Headquarters map
Let the AI kill you 5 times
Kill the AI
Get the achievement
Lose some respect and self-esteem 'cos you got it like a n00b