[Astrohawke]

It's all random. Killing speed doesn't matter. You just have to spend a lot of time on it. I spent a solid week doing this and I came out with an average of 2 plugs a day. Some days I got none and on a good day I got 4. Sometimes I kill 5 and 1 will drop. Other times I didn't get anything for 8 hours (over 2 days). From what I've found though, on average one drops every 2-3 hours. If you're not serious about investing a lot of time into this then I suggest you don't do it. If you're just gonna try for 30min-1hr, get bored and then complain about not getting anything, then don't waste your time and just keep leveling.