This one can be done one of two ways. One I would consider "boosting" the other is not.
Just play the game naturally online. Play as one character over and over again and get good with him enough to get a few kills per match. On an average game even a lower skilled person will get at least 3-4 kills. Easy characters are Clunk, Lonestar, Skolldir (unlocks at level 11) & Derpl (unlocks at level 15). All of these characters have a large amount of health and have relatively simple attacks. Just keep playing and you'll get this within some time
Pick to play a private match. Press the square button to lower the AI difficult down to the lowest. The AI becomes extremely aggressive, brave, and even stupid. The computer will only run away when they are at 20 or less health (2-3 hits from a normal attack or one special attack from most characters). In all reality a person who skilled at MOBA's or at the game itself will get at least 10-15 kills in a match, and someone who is not will likely still get 5-10. I managed to go 22/1 in one game as Clunk.