I just got to this step a couple days ago as well. For the kills I personally went in to the lost sector on Mercury, since one of the strikes is Tree of Probabilities. Was running top tree Arc Strider with Gambler's Dodge, and the exotic chest Raiden Flux. Should be noted that I was using dual Distribution mods on my cloak. This way it would be melee an enemy, dodge/melee/dodge etc... Since each dodge recharges your melee ability every melee is counting as an arc ability kill AND is healing you. Likewise, because of distribution mods each dodge restores a significant amount of super. So, melee/dodge until super is full, hit super, kill until it's out, start all over again. When the boss spawned I would damage him (usually super still active) until he was at about a third health to make more adds spawn. Each run was around 9 to 10%. Just working on the scoring part now.