The Ropalolyst itself isn't a particularly bad farm, maybe find a good squad to run with?
I'd even say that Wisp is one of the faster ones to get, at an average 25% chance per part per run, and in a decently fast and consistent mission.
Frames like Harrow, Ivara, Equinox, Protea, Octavia, Gauss, Khora, Protea, etc. all have worse drop rates from harder/longer missions.
Stalker is by far the worst example of RNG, but there aren't many examples like him. There's no need to farm him either, as all of his gear is (literally) equal to, if not surpassed by primes.