When i went for 500 kills, i wanted all boss kills as well as the kill count. I did Endurance from 1 to a really bad boss (e.g avatar). Then repeat, remember if you have class A boss left, try and get above endurance lvl 7 before resetting, cos thats where class A bosses can only appear. Once you have the specific boss kills you want/need, do either climber or endurance till you meet a shit boss, reset and repeat.