This quest seems impossible in the current sandbox and meta. The rapid kills are impossible without extreme luck if you kill 2 people with 1 grenade. Everything else is possible but that requirement. You have a extremely short amount of time to get another kill since its less then 3 seconds. This needs to be addressed since pinnacle weapons will always be available and the meta will keep changing and so will ttk removing any chance a grenade launcher will get 2 rapid kills
Hopefully all of you upvote this post so maybe a dev could respond and at least discuses this with the dev team to see how they can change the rapid kills into this new fast ttk sandbox. If you have something negative speak your mind or any other solutions
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You have to do that part of the quest with a heavy launcher and just spam it when you see two or more people close to each other.