Question: Are you sure the first line shouldn't be "Tree beats Earth"? The first two lines just seem a little weird to me, particularly since it gives Fire two strengths and two weaknesses (one of which makes very little sense).
I assume there would be some way of strategically increasing a given card's attack/HP, whether using evolution/sacrifice (like the Pokemon and YGO TCGs) or power-up cards/effects. Otherwise it would be a fairly stagnant game, with everyone just stacking their decks with 6/6 cards.
As for "a card with 6 attack is unbeatable," that could be a little dangerous. You could probably make good use of cards that weaken other cards; otherwise, barring an unlikely type disadvantage against a fellow 6-attack card, the game would pretty much be over.
For the time being, you might consider one other thing: what is the object of the game? Is it like War, where you simply want to defeat all your opponent's cards? Is there a limit on how many cards you can play, or some benefit from eradicating your opponent's cards? Gameplay obviously involves defeating your opponent's creatures, but how does a player claim victory?