Reached battle 25 and the medium badge to reach battle 20 hasn't triggered. Maybe that's why is harder than the easy badge of getting to battle 30 and opening the arena?
Not a bad game, I quite like pootling around trying to pull off clever ricochet shots and failing. I feel that Kongregate has done them a disservice with the badge of the day tactics though.
May be a bit late, but I found out that a boost kill was a killshot done while using the speedboost.