As base, it fires 4 homing missiles. So, multishot and firerate are enough to apply stats. So

-Vigilante Set to boost primary criticals
-Vile Acceleration+Vigilante Fervor+Terminal Velocity to improve speed
-Vigilante armaments also contributes to apply stats (multishot).
-Primed Firestorm to apply stats (massive area).

Instead a riven, use 60/60 elemental.

Best rolls for riven could be fire rate or elementals to alternate with primaries or melee (as second chance proj speed); with P. [Firestorm]: 4.98m area.

However, gas stat apply area effect, so maximize the chances to apply this status effect over group of enemies.