Played well, Void Zephyr can be soloed by one melee DPS on a decent setup (MUB Ifrit or better, any matching 4* Dull Res or non-Void 5* Elemental weapon, preferably a Sword character). Verica is fine for learning the fight and helping lesser skilled players clear more consistently, which is as far as most players bother to go. However, for someone who has farmed Zephyr as much as I have, I detest seeing her in my lobbies - at best she denies Marth's passive if I'm screwing up, at worst she does less DPS than an AI on my bench.
I've been around the block in terms of compositions, from learning the fight on Dull Res Naveed, to having a Solo Auto team with three MUB Dull Res weapons (Karl Phoenix lead, Marth/Naveed DPS, Verica), to my current composition of Co-Op Marth on a MUB 5* Elemental with Sinoa (and sometimes Xania if first join has an empty bench) on Dragon Delay Res and Dragon Prep.
I actually clear faster with a dead Co-Op corpse or two than I do when that player quits early. When Co-Op converts to Solo, the Co-Op AI will cause me to lose Dragon Gauge, which occasionally means I can't Dragon in time to avoid a Dull during Overdrive. I can still clear, but it turns a 2:30+ run into a 2:00 run.
Just to give an idea of how much I've farmed Void Zephyr/Agni, I'm sitting on ~50 Void Seeds and ~3000 Void Leaf, with the entire stock of Bronze and Gold Fafnirs purchased. Zero Squishums, but that's not the main reason I'm farming.