Ranged witch and spellblade

[Desmodeus]

Can someone give advice how it will be better to make ranged builds of witch Serafen and spellblade Aloth for mass shooting squad?  Is shooting even viable for this multiclasses?

[Theosupus]

If going Ranged I like Fire in the hole or The Red hand or Veilpeircer for Serafen and Veilpeircer or Spearcaster for Aloth (with maxed arcana for spearcaster).
Shooting is very viable.
I dislike single damage type weapons generally (because I'm lazy and don't want to switch weapons per encounter) so I tend to like the bows over the guns, usually.
With that said, opening w/ double-shot (shooting from stealth let's you shoot a second time instantly) only works with reload weapons is very powerful. Also heavy armor doesn't impact reload like it does recovery so you can armor up if you go with reload weapons w/o impacting DPS as much. 

I love veilpiercer due to its silencing proc.
For Arquebuses I like the red hand's "No rest for the wicked" & "Guilty conscience" (which are stackable if going for a no rest run) but the short range can be annoying.
 

[Boeroer]

With a Witch both Veilpiercer as well as St. Omaku's Mercy are viable because of their 50% chance to skip recovery on crit. Borrowed Instinct + some crit conversion from Barb in combination with attack speed and Blood Thirst let you shot quickly.

Hunting Bows now also work well with the modal, especially with Borrowed Instinct.

Guns and crossbows are a bit suboptimal because Blood Thirst doesn't work with them.

I also like to make Serafen a SC Barb and go ranged with a bow, aiming for Driving Roar later which is an extremely good ranged dmg/cc ability (given its low cost).

Spellblade: any AoE weapons like mortars or Minor Blights are great when you want to apply stuff like Toxic Strike to many enemies at once. Infuse with Vital Essence makes the AoE bigger. Alacrity speeds you up, Combusting Wounds amplifies the AoE dmg...

Blightheart is also good on a spellblade. One of the best ranged single target dmg weapons in terms of dmg is Nannasin's Cobra Strike - but its range is very short, like a reach weapon basically. But it is a ranged weapon (with high base dmg and a nasty DoT). 

[Desmodeus]

Should I go for dualwielding in case of mortars and pistols? Or one handed will work better for accuracy bonus?

[Boeroer]

You mean with a Spellblade? With mortars I would prefer dual because both mortars complement each other very nicely: Hand Mortar + Blinding Smoke unlocks Deathblows and then Fire in the Hole with Chain Shot gets all the nice Deathblow bonus. Also: Full Attacks are just better with dual mortars and the Spellblade has plenty of Full Attacks.

Generally pistol + modal is better than dual pistols when it comes to auto-attacks. But again: Full Attacks like dual weapons.
But imo mortars are a lot better than pistols on a Spellblade in general. Also because of dual damage.

I like pistol + modal on chars that don't use Full Attacks - like Ciphers, Chanters... I would always go for Eccea's Arcane Blaster at least as backup weapon because pierce-only weapons can be so frustrating.