I was bored so I made up a list of the best and worst elements to use in each coliseum venue, based on the frequency of opposing elemental enemies in each venue (also accounting for the elements of bosses). Each list is in relative order from most good (for best) or bad (for worst).
Golem Worskshop
Best: arcane, wind, earth, lightning
Worst: light, water, fire ice
Kelp Beds
Best: plague, lightning, light, ice
Worst: wind, fire, water, earth
Mire
Best: earth, light, fire, lightning
Worst: nature, wind
Ghostlight Ruins
Best: plague
Worst: earth, wind
Harpy's Roost
Best: wind, nature, water
Worst: lightning, light, ice
Crystal Pools
Best: nature, plague, arcane
Worst: wind, fire
Boreal Woods
Best: shadow, fire/lightning
Worst: water, nature/plague
Rainsong Jungle
Best: nature, plague, wind
Worst: arcane, light
Volcanic Vents
Best: earth, water, wind
Worst: ice/plague, lightning, arcane, light
Arena
Best: lightning, arcane, ice/fire, nature
Worst: shadow, plague, wind, water
Waterway
Best: ice, nature, plague
Worst: fire, light, arcane
Redrock Cove
Best: plague, lightning, arcane
Worst: fire, wind, water, earth
Bamboo Falls
Best: water, ice/nature, plague
Worst: earth, lightning, fire
Forgotten Cave
Best: water, nature, plague
Worst: fire, arcane
Sandswept Delta
Best: arcane, lightning, nature, light
Worst: wind, ice, fire, maybe shadow
Scorched Forest
Best: earth, light, fire
Worst: nature, maybe plague
Woodland Path
Best: fire, ice, plague, wind
Worst: arcane, water, nature
Training Fields *has 2 of all elements, rankings based on encounter chance
Best: nature, earth, fire, wind
Worst: light, arcane, plague, lightning
Elements with the most Bests: nature, plague, lightning
Least Bests: shadow, light, water
Most #1 Bests: plague/earth, nature
No #1 Bests: light
Element with the most Worsts: fire, water/wind, arcane/light
Least Worsts: shadow, plague, lightning, ice/nature/earth
Most #1 Worsts: fire/wind
Least #1 Worsts: light (only Training Fields)
Here's a chart showing which elements are strong/weak against others
Golem Worskshop
Best: arcane, wind, earth, lightning
Worst: light, water, fire ice
Kelp Beds
Best: plague, lightning, light, ice
Worst: wind, fire, water, earth
Mire
Best: earth, light, fire, lightning
Worst: nature, wind
Ghostlight Ruins
Best: plague
Worst: earth, wind
Harpy's Roost
Best: wind, nature, water
Worst: lightning, light, ice
Crystal Pools
Best: nature, plague, arcane
Worst: wind, fire
Boreal Woods
Best: shadow, fire/lightning
Worst: water, nature/plague
Rainsong Jungle
Best: nature, plague, wind
Worst: arcane, light
Volcanic Vents
Best: earth, water, wind
Worst: ice/plague, lightning, arcane, light
Arena
Best: lightning, arcane, ice/fire, nature
Worst: shadow, plague, wind, water
Waterway
Best: ice, nature, plague
Worst: fire, light, arcane
Redrock Cove
Best: plague, lightning, arcane
Worst: fire, wind, water, earth
Bamboo Falls
Best: water, ice/nature, plague
Worst: earth, lightning, fire
Forgotten Cave
Best: water, nature, plague
Worst: fire, arcane
Sandswept Delta
Best: arcane, lightning, nature, light
Worst: wind, ice, fire, maybe shadow
Scorched Forest
Best: earth, light, fire
Worst: nature, maybe plague
Woodland Path
Best: fire, ice, plague, wind
Worst: arcane, water, nature
Training Fields *has 2 of all elements, rankings based on encounter chance
Best: nature, earth, fire, wind
Worst: light, arcane, plague, lightning
Elements with the most Bests: nature, plague, lightning
Least Bests: shadow, light, water
Most #1 Bests: plague/earth, nature
No #1 Bests: light
Element with the most Worsts: fire, water/wind, arcane/light
Least Worsts: shadow, plague, lightning, ice/nature/earth
Most #1 Worsts: fire/wind
Least #1 Worsts: light (only Training Fields)
Here's a chart showing which elements are strong/weak against others