Elemental synergies

Aether

Fractal has a good synergy with Giant Frog because it's very cheap and has an awesome attack/cost ratio.

Air

Darkness

Fractal can be used on Devourers to absorb the opponent's quanta.

Devourer and Gargoyle use Earth quanta for their abilities. Earthquake and Devourer work together to deny quanta.

Pandemonium (unupped) can be used with Voodoo Dolls for extra damage.

Rage Potion can be used on Voodoo Dolls for double damage.

Ghost of the Past is an excellent target for Nightmare.

Death

Sparks cause death effects that benefit Death.

Flesh Spider can use web to tie down opposing creatures while Wings provides defense.

Nightfall boosts Death creatures, so Deathstalker can inflict 2 poison per turn.

Schrodinger's Cat can cause repeated death effects.

Otyugh can kill enemy creatures by devouring it and trigger all death effects.

Graveyard can create many Skeleton's that Reverse Time will turn into any creature. Mummy is a cheaper version of the Pharaoh so you can swarm Scarab's faster.

Chrysaora can poison the opponent with Death quanta. Flooding can cause mass death effects.

Earth

Entropy

Fire

Gravity

Life

Fractal has a good synergy with Giant Frog because it's very cheap and has an awesome attack/cost ratio.

Light

Time

Water

Mind Flayer forms a direct link with aether. In addition, a bolt stall can be done with Dim Shields and Ice Bolts.

Toadfish, though not it's upgraded version, uses air for a creature control ability, and there is a powerful control combo in combining Freeze (and Arctic Squid) with Shockwave

Strong but fragile attackers, as well as Arctic Squid and Ice Shield for control, can be hidden under Cloak to ensure their survival long enough to pose a threat.

Chrysaora is often used with Poison as a formidable source of center damage. Freeze and Ice Shield can also be used in conjunction with Plague to kill creatures without them attacking.

The weapon Trident uses earth for pillar removal.

Nova can be used alongside water's many creatures that use other elements to form a versatile water-rainbow base.

Fire is used to power Steam Machine, both through it's ability and, especially upgraded, with the buffing power of Rage Potion.

Freeze enhances the damage done by Catapult, which works especially well with gravity's high HP creatures, such as Armagio.

Forest Spirit use water for growth. Beyond that, Adrenaline, including using Nymph's Tears to gain Green Nymphs, can be used with Puffer Fish for decent damage

Ice Bolt can be used with Sanctuaries and Miracles to create a deck that stalls until it can defeat the opponent with Ice Bolt's spell damage and Morning Star for immortal damage.

Nymph's Tears can be used to create Golden Nymphs, which provide stable damage and draw acceleration. Also, Eternity can be combined with Freeze and Ice Shield for a powerful defense.