The easiest way to obtain this is to stay down on the bottom level and let the enemies come to you. It is by far the easiest way to progress in levels, however the highest I got using this method is level 8 as it gets way too challenging after that level.
Alternatively you can watch the video below on the best locations to go to according to each level 1-100!
Note: If you are going to vote negatively please comment or PM me first as I am willing to adjust anything that needs to be for this to be proper.