This game really made me angry. If you accidentally right-click while shooting (god forbid) it sits there and does nothing until you die. And not only that, but you can't do anything else with your browser short of ending the task. I don't dare try for more achievements. I'll punch a hole in my computer by the end of the night. =X
The repeatable exp kicks in around.. i think boss 20? Somewhere around there. Until then you get EXP only on the first kill.