However, given it's additional flat damage affected by critical multipier that can proc status (I believe ?), you can exploit :
High critical multiplier to maximise the value of the flat damage buff, with critical chance for reliability.
Large direct AoE and/or punchthrough. A rocket launcher (Ogris for example) is a small projectile that produce AoE on impact, meaning the small projectile will benefit from the electricity damage but not the AoE. What you want is something like the Arca Plasmor that directly shoot a big projectile.
High status chance, potentially above 100% to get multiple procs from the same shot, ideally with status duration as well.
Faction damage (including Roar) for double buff on electricity status.
Damage buff based on the intial hit, which can either be Toxin Lash (exclusive to Saryn) or Xata Whisper, which is the only available choice here.
Movement speed ability to charge the buff faster, which include Molt, Infested Mobility and possibly more.
Based on these, without deeper research I would advise either something like Catchmoon or or a beam that trade raw DPS for critical/status chance and a high Riven disposition to maximise critical and status stats. With this, Roar is probably the way to go (if it work), unless you prefer speed abilities to charge each shot faster.
Also keep in mind only one projectile will get the buff, so multishot (both innate or from mods) is not recommanded, leaving more room for other mods.
Also make sure you test all of that to make sure it's true, because I haven't tested all of these myself recently so I can't guarantee it's still true.
It gives passive health regen to your Operator, not Warframe.
It can be confusing because it say it's doubled while controlling your Warframe, but what is doubled is the Operator health regen.
lukinu_u's post in Does Volt's passive work with Subsumed abilities, and if so what are some good synergies? was marked as the answer