I think one ability or mechanic is too small to contain more than a 3rd of an element. As such I denote elements by their core and 3 focuses.
Aether
Aether is space. It has dominion over electricity and otherworldly beings. I think of Aether as the element of alternative realities. Aether uses this core to focus on immortality (Immortal, Dimension Shield, Phase Dragon, Quintessence and Aether Nymph), card generation (Fractal, Twin Universe and Mindgate) and skill removal (Lobotomizer and Psionic Wave).Air
Air is the sky. It has dominion over storms and the birds in the sky. I think of Air as the element of maneuverability. Air uses this core to focus on creature control (Owl’s Eye, Thunderstorm, Unstable Gas and Shockwave), versatile attacking (Dive and Inflate) and animation (Flying Weapon). Both versatile attacking and animation are lacking in cards.Darkness
Darkness is the night. It has dominion over the denizens of the night. I think of Darkness as the element of parasitism. Darkness uses this core to focus on vampirism (Drain Life, Minor Vampire, Liquid Shadow, Vampire Stiletto and Black Nymph), resource theft (Pest and Improved Steal) and reflecting the opponent’s strength (Improve Steal and Voodoo Doll).Death
Death is the end. It has dominion over the dead and decay. I think of Death as the element of transformation. Death focuses on causing death and directing the death to cause advantage. Death uses this core to focus on poison (Virus, Plague, Poison, Deathstalker, Aresnic and Alfatoxin), death triggers (Scavenger, Bonewall and Graveyard) and replacement (Alfatoxin and Graveyard).Earth
Earth is the ground. It has dominion over rocks, minerals and burrowing creatures. I think of Earth as the element of durability and strength. Earth uses this core to focus on health (Stone Form, Plate Armor, Stone Skin, Basilisk Blood and Auburn Nymph), protection (Burrow and Enchant Artifact) and permanent destruction (Earthquake and Pulverizer).
Entropy
Entropy is chaos. It has dominion over random chance and abnormal beings. I think of Entropy as the element of luck both good and bad. Entropy uses this core to focus on creature improvement (Mutation, Fallen Elf and Chaos Power), random hostility (Chaos Seed and Pandemonium) and disruption (Discord, Butterfly Effect, Antimatter and Purple Nymph).
Fire
Fire is the flames. It has dominion over heat and rage. I think of Fire as the element of consumption. Fire uses this core to focus on creature destruction (Fire Bolt, Fire Shield, Rain of Fire, Immolation, Rage Potion, Red Nymph), permanent destruction (Deflagration) and cheap offense (Ash eater, Crimson Dragon, Fire Bolt, Lava Golem, Rage Potion, Phoenix, Fahrenheit).
Gravity
Gravity is a force. Gravity has dominion over momentum and massive creatures. I think of Gravity as the element of pulling forces. Gravity uses this core to focus on massive creatures (Armagio, Colossal Dragon, Chimera and Titan), creature direction (Momentum, Gravity Pull, Gravity Shield and Sapphire Charger) and draining resources (Oytugh, Black Hole and Amber Nymph).
Life
Life is a jungle. Life has dominion over land creatures. I think of Life as the element of growth. Life uses this core to focus on cheap efficient creatures (Firefly[skill], Horned Frog, Cockatrice and Adrenaline), Healing (Druidic Staff, Empathetic Bond and Heal) and Poison (Forest Scorpion and Thorn Carapace).
Light
Light is illumination. Light has dominion over light emitters and angelic beings. I think of light as the element of holy light. Light uses this core to focus on generating light (Photon, Holy light, Luciferin, Hope and White Nymph), healing (Holy light, Angel and Miracle) and angelic creatures (Golden Dragon, Pegasus, Angel).
Time
Time is like a river. Time has dominion over time travel and ancient creatures (mostly from ancient Egypt). I think of time as the element of time compression and stretching. Time uses this core to focus on acceleration (Fate Egg, Evolve, Golden Hourglass, Precognition, Sundial and Golden nymph), deceleration (Procrastination and Sundial) and rewind (Reverse Time and Eternity).
Water
Water is the ocean. Water has dominion over creatures of the sea and liquids. I think of Water as the element of reflection. Water uses this core to focus on freezing (Freeze, Ice Bolt, Arctic Squid and Ice Shield), synergy (Chrysaora, Toadfish, Mindflayer, Steam Machine, Trident, Forest Spirit) and personification (Nymph’s Tear and Nymph Queen).
Other
To me Other has three roles. First it can act as parts of all elements combined together. Second it can act as unifying traits of all elements. Third it can act as characteristics not exhibited in any element.
The first would be a card that responds to different elemental stimuli in different ways
The second would be a card that could fit in every element
The third is a card that could not fit in any element
