My suggestion (assigning each element to a zodiac sign, and make groups according to astrology), matching Cardinal to astrological cardinal signs, material to astrological fixed signs and spiritual to astrological mutable signs.
Fire Element signs :Aries (Cardinal) : Entropy
Leo (Fixed) : Fire
Sagittarius (Mutable) : Light
(Fire generates Light and increases Entropy of the system)
Earth signs :Taurus (Fixed) : Earth
Virgo (Mutable) : Death
Capricorn (Cardinal) : Gravity
(Earth has gravity and dead creatures return to the earth)
Air signs :Gemini (Mutable) : Life
Libra (Cardinal) : Aether
Aquarius (Fixed) : Air
(Air is the most necessary for living out of all the basic four elements, and aether is the same for the universe as the air if for the living)
Water signs :Cancer (Cardinal) : Time
Scorpio (Fixed) : Water
Pisces (Mutable) : Darkness
(There is darkness in deep water. Time flows, like water)
This gives us three groups based on the purpose of the element (Cardinal, Material, Spiritual) the same way as was already suggested, while also providing the four groups based on the main element of the group (Fire,Water,Earth and Air)
(
http://www.screenshotdumpster.com/view/m7Gbd53981/elements)
Organizing the elements like this could give us seven possible multi-element pillars, making deck construction much more interesting and opening way more choices than only three or four groups, while keeping the best out of both type of suggestion.
Also note that in astrology, opposing elements are ones that "need" each other for their existence, kinda like the two sides of a coin, so they aren't the natural enemies of each other.
For example, Fire opposes Air, because Fire is necessary for matter to became aerial (otherwise it would be frozen and become solid), while Fire also can't exist without air (oxigen is needed for a fire to form)
So the original assigned opposing elements (Fire to Water, Earth to Air, do not fit in this system. Here, elements that are harmful for each other are the other two elements of the same group, for example, in Fire's case, it's water and earth, not the opposing Air)
This system fits well here, making Life and Light being enemies to both Darkness and Water, Fire and Air to Water and Earth, and finally Aether and Entropy (freedom,chaos) to Time and Gravity (order,control).