[Fern]

To keep my thoughts concise, I believe one of the biggest contributing factors is that nearly no lightning-resist itemization exists, and Splash exists.

Every other element usually has some combination of boots, gloves, accessory, or torso pieces to get (up to) 30% resistance.  Lightning has Raijin and a shield, both of which cannot be used by most setups, and a half-effectiveness torso in Cloak of Many Colors.  Splash mitigates the latter entirely, as does Indignant Idol.

Indignant Idol is its own can of worms with regard to effectiveness, but I would recommend first adding a pair of 10-30% lightning resist boots, and a 10-30% lightning resist accessory.  If this proves insufficient, I would limit Splash or Idol's ability to throw people into negatives in some manner before biting at the heels of an invocation.

This pretty much sums up my main thoughts on the matter. The setup is strong, yes, but it's not absolutely strong to the point it's standard arena PVP fare (at least from what I've seen lurking there.) This combination has existed for a few years already, but back in the day it wasn't as feasible as it is now thanks to Silence being more common (though, now Silence is pretty much a rarity, specially when the few accessible sources tend to be shut down by most mages packing a Heron Feather.)
 
It's only stronger now because of Silence being a rarity and Indignant Idol's existence as a meta mage hand slot with little to no drawback. If there were more resistance items dedicated to giving lightning resistance, then there wouldn't be any need to nerf hammer a bunch of things that while fearsome when combined- are still used individually by other setups that aren't trying to maximize lightning cheese.
 
It's better to add lightning resistance items, so that not only there's more equipment to play with, but future lightning stuff gets future-proofed by it existing as a sort of collateral effect, instead of killing a set of skills and throwing several setups unrelated to the Mist+Splash+Idol+Custom Spell build into the gutter.