They do. I am actually surprised they have not nerffed the ripper. But overall it's tough getting the 6 kill challenges with the weapons. You only have your specialist ability thee times a game Much of it is just dumb luck. IE playing on a smaller map and having the enemies lined up, not getting shot and killed within 5 seconds.
Based off of some hints given to me, may I suggest overdrive and hardpoint or safeguard? Or ground war version of safeguard?
Seems to let you get your specialist ability once a round or more if you wreck, and since you can spread the 6 kills out over the rounds (most safeguard games go 4 rounds if teams are somewhat even) it makes it a bit easier?
I say this having only just unlocked hero gear and only unlocked ruin's helmet.