This cheevo will be awarded when you get your 15th kill of an enforcer with the killing shot being with the spammer.
You will get the spammer at the end of episode 2 once you defeat kid xmas.
So from episode 3 onwards every time you see an enforcer try to damage him as much as you dare to using normal attacks and once you think his health is sufficiently low then finish him of with the spammer.
It does not matter of you use the normal spammer or junk shot as long as the killing blow is from the spammer it will count.
Also be aware that however many kills you get before a checkpoint are not save if you die, once you make a kill you must get to the next checkpoint for it to count to your total.
As per the comments section overloads definitely do NOT count towards this cheevo.