The Thundercrack Carnivale starts this Sunday!
Places to farm for Lightning Holiday currency. Non-italicised are Lightning, italicised are Neutral. Only areas with Lightning enemies are listed.
Training Fields - Emerald Webwing; Amaranth Moth
Scorched Forest - Cockatrice; Red-Winged Owlcat
Sandswept Delta - Storm Seeker; Peacock Scorpion
Forgotten Cave - Emerald Cave Jewel; Crimson Reef Snail
Bamboo Falls - Baku; Tengu
Waterway - Shale Thresher; Radioactive Slime
Boreal Wood - Squall Rasa; Winter Wolf
Harpy’s Roost - Blue Tang Hippogriff, Storm Seeker; Dodo
Mire - Common Podid; Shellion
Kelp Beds - Maren Warrior; Mammertee
Looks like your best bet is Harpy’s Roost (and Arena of course, even without any Lightning enemies there are enough Neutrals).