I believe gloams were stronger pre-1.13
Since 1.13 +% Damage from Champion and the Extra Strong Unique bonuses is halved then rounded down when applied to Willowisp physical melee attack damage, but their lightning bolt damage remains unchanged: those of Unique Willowisps and their minions in level 85 areas apply 200-300 base lightning damage, increasing by 1/16 with each additional player (maximum 287.5-431.25 at /players 8). As always there's 5% chance of a critical hit doubling damage, and non-physical damage added by Unique or Champion bonuses is also added to bolts.
Aside from equipping Lightning Resist, +% Maximum Lightning Resist and Lightning Absorb (the latter two aren't really necessary though), make sure you aren't standing next to a mercenary or pet when you're getting fired upon: the bolts have Last Collide enabled, which normally prevents them from colliding with their last target again, but the bolts are large enough that they can collide with an adjacent target and then collide with their previous target again (since it's no longer their last target) before moving out of range.
After all you're not even going to see Gloams most runs.
Black Souls spawn in the Worldstone Keep Level 2 ~48% (13/27) of the time, while Burning Souls spawn in the Throne of Destruction ~38% (17/45) of the time; at least one of them will spawn in those two areas ~67.7% of the time.