TL-DR: Line up enemies for shots

Alternate mods

Corrosive version works well against Grineer and Orokin, and to fit both you'd be using an additional forma.

You can also swap Vigilant Supplies with [Fatal Acceleration] for one less forma but I would still highly recommend spending that forma due to the poor ammo economy of [Phantasma]. If you're all too trigger happy then [Primed Shotgun Ammo Mutation] is best since the DPS gain from VS is actually minimal at best.

Riven recommendation

Damage, Tox, Status.

Additional usage notes

[Phantasma] is somehow coded both as a shotgun and a beam, so having 100% status chance before multishot is necessary.

Line up enemies and fire. The innate punchthrough should be sufficient to hit multiple despite the falloff.

Video is showing corrosive version, but similar playstyle and performance should be expected.