Earth has 42 Creature cards, the most of any element, with 14 Rare and 2 Extra. The Culdcept BEGINS best earth cards are led by Dark Master, and the reason Earth shows up in so many competitive Books is the Support package underneath it.

Culdcept BEGINS Best Earth Cards: Rare and Extra

Cost is shown with its deployment requirement; AT and HP are the printed values.

CardCost and requirementAT / HPWhat it does
Dark Master120+ G, Earth land, discard60 / 80/80Scroll Critical Hit, Boost [Earth] AT+10; the user loses 100 magic at the end of the battle
CarbunflyExtra, 100+ G, Fire land40 / 40/40Attacks First, Reflects Scroll Attacks, and revives as a Carbuncle if it is destroyed
GoobaExtra, 0 G0 / 10/10A free Creature that Gooba Queen can deploy onto an empty territory
Kudzu Folk80 G30 / 30/30If it is destroyed while you hold four or fewer territories it revives as a Green Mold; Defensive and Regeneration
Vampire60+ G, Earth land40 / 40/40Attacks First, and on a successful attack its HP grows by the damage it dealt
Mandrake60+ G, Earth land10 / 50/50If its HP is 20 or under when the battle ends, both it and the opposing creature are destroyed
Sakuya60+ G, Earth land20 / 30/30Support, and a Territory Ability that converts one of your territories to Earth for 120 G
Humbaba50+ G, Earth land, discard50 / 40/40Boost [Air] AT-10, and on a successful attack it adds Poison to the enemy creature
Shambling Vine50+ G, Earth land20 / 40/40Penetrates against Fire and Earth, gains AT equal to your Earth territories ×10, and gains 20 HP if either creature carries an enchantment
T'ao T'ieh50+ G, Earth land40 / 40/40On victory its base AT, HP and MHP all rise by 20
Basilisk50+ G, Earth land30 / 40/40A 40% Instant Death, and at the end of the battle it lowers the level of the territory by one
Coati30+ G, Earth land30 / 30/30In battle it gains AT equal to the number of Earth creatures in play ×5
Gooba Queen30+ G, Earth land10 / 40/40A Territory Ability that deploys a Gooba onto a vacant territory of your choice for 20 G
Odradek30 G20 / 20/20Support, and its HP and MHP cannot be altered by spells
Sage50 G20 / 30/30Support, and immune to Destroy Item and Steal Item effects

Two of them are cheap and change the board on their own: Gooba costs nothing and can be deployed onto an empty territory, and Sakuya converts one of your lands to Earth for 120 G instead of making you fight for it.

Earth's Support Package

Earth is the element with the strongest Support package — Dryad, Great Tusker, Woodfolk, Sage, Odradek and Sakuya — which is why Earth appears in most Fire/Earth competitive lists. Support Creatures buff the Creature they support and can use creature cards from your hand as items, so they are worth slots of their own. Deck building covers how to give each of them a job inside 40 slots.

Why Earth Shows Up in Fire/Earth Books

Two elements that each bring their own support Creatures can support themselves, which is exactly what Fire and Earth do. Best element combinations explains why that pairing is the standard opening choice.