AoE cannot trigger head shots or work with Aptitude. Just go for a balance of big, consistent crits.

The Lenz has a built-in "[Arrow Mutation]" effect, so you don't need [Arrow Mutation] or [Vigilante Supplies] in the Exilus slot. [Terminal Velocity] matches the default polarity, saving you a forma, and will improve projectile accuracy a bit. It is, however, highly worth the extra forma for [Cautious Shot] as any close range hits or clips with this will knock you on your ass.

Updated to use Primary [Frostbite] for maximal output. Overframe does not accurately [Reflect] the gains from this Arcane with [Hunter Munitions]. The net gain is that it will be increasing the number of slash procs (via multishot) and the severity of procs (critical multiplier) with every target hit by the AoE and forced cold proc.

If you want to target a specific faction, replace [Serration] with a (Primed) Bane mod.