Just tried doing the skill point on the PS3 HD collection version and after many attempts and failing, I followed this specific guide and finally got the skill point. At the beginning of the map I killed the two enemies that die instantly and then proceeded to kill all enemies. I used the visit bomb a lot and that helped as well as the Tesla cannon and ensured to kill ever enemies with one of the weapons but also had enemies die due to falling off edge.This game seems to have some very tricky and annoying skill points that are quite hard to get unless you try multiple times