Okay, nice idea. I approve of this. I've played another online strategy card game called urban_rivals and this hero idea is similar to the leaders they have. It's a very successful game and people like the hero idea. They make a lot of money to from their card packs... I have to admit I've probably spent about a $100 on that game. The random card packs just made it too addictive. I because rich fast... and so did they.
My problems with the cards:
1) Overpowered
2) The name (Heroes)
Now to address the first problem (Overpowered). The cards cost doesn't negate the fact that they have insane power and life. Most of the effects seem fine, but a person needs a chance to be able to kill the card. That's why it would be useful for you to an anubis out to immortal it fast or something along those lines. Blessing it a few times or adding a heavy armor too. That way a person has to think more when building their new deck. Instead of making everything revolve around the cool new ability they'll have to worry about keeping it alive too. It also makes it less of a game breaker. I think it depends on the type. The air "hero" (I'll talk about that later, but I'll use this for now) shouldn't have a power or life rating. I'll go through what I think about what I think of each of your cards.
Fire (Bazuul): 12/18. 20 fire quantams. Way too weak and powerful at the same time. Fire creatures have low life and high attack. If it's stronger than the rest it should be more pumped up. I think it should be a 20/8. Still killable, but it makes sense. Personally I don't like the ability. It seems to flow a lot more into the magic arena. Fire, in Elements, is more of an aggressive element. Instead of your ability I think it should have Inferno: +4/+0. This card would still be great, but not too overpowered. It could still be killed by multiple combinations and yet it's still worth it's cost. You can combine fire and gravity for a little momentum action. Think of a card that'll probably take two turns to kill which will do a guaranteed 21 damage first turn? Pretty sick in my opinion.
Light (The Chosen One): 0/24. 20 light quantams. This card is great in my opinion. There's only one point I'd like to bring up. My first point is that it has too much life. I think it should be a 0/16 instead of a 0/24. Now you might be thinking that is a huge drop, but not really. You have to remember that light has Guardian Angels... which heal. It'll still be hard to kill, but not impossible. Other than that it's fine. At first I was thinking it's ability should have a cost, but light isn't that strong anyway. Many decks can kill the creatures while their getting strong. If "The Chosen One" is left by himself than it's not a big deal what his ability does.
Time (The Clockmaster): 0/10. 20 time quantams. Great card. I have no problems with it being overpowered. In fact I'd suggest it became a little stronger. The ability is PERFECT, but I'd like to see some attack power. I'd suggest it being a 3/10.
Entropy (The Joker): 10/16. 20 entropy quantams. I like the idea of this ability, but honestly it wouldn't really be helpful in an Entropy deck. I think it should have another ability. Anarchy: Whenever this creature deals damage to an opponent the damage is taken from the opponents quantams randomly. You then receive the same amount of random quantams as the amount taken from your opponent. I'd suggest it being a 9/12 also.
Earth (Rocky): 16/20. 20 earth quantams. Okay, this guy is just a beast... and not in a good way. He's FAR too overpowered. Not only that, but it's obvious the ability came from MTG. Taking other earth creatures as an example I would suggest this card being a 15/16. The ability is too overpowered and screams MTG. Also, it's a little debatable to say what's an air creature or not (a Spark floats right?). For the ability I'd suggest this. Aftershock: For every pillar or tower an opponent loses your opponent takes 5 damage. Now this ability would be great for an Earth deck. Poseidon and Earthquakes make this card deadly, but reasonable.
Darkness (Vampire Lord): 12/16. 20 darkness quantams. I like the art on this card, good job on finding it Scaredgirl. Great ability and I don't have any complaints for it... well instead of "loses" I'd suggest using the word "drains." I'd suggest it started off weaker though. It basically has growth already, so it would be problematic to kill for most decks. I'd suggest it starting off as a 8/6. That's double an upgraded Vampire. It seems pretty fair to me. Also, I'd suggest lowering the cost to only two darkness. Being overpowered and underpowered matter.
Death (Zombie King): 10/18. 20 death quantams. The ability is perfect. It fits into the death flow and it's reasonable. I'd only make one change. Instead of "starting off" with an infection the Zombie King should get infected each turn. You would even be able to keep the power and life as it is and it would still be pretty fair. So basically every time is does a mass infection it gets infected too.
Gravity (The Machine): 5/50. 20 gravity quantams. Besides the fact you put "you" instead of "your" this card is a bit overpowered. I think the ability should be this. Propulsion: All creatures on your field have momentum while The Machine is out on the field. That way when it dies the momentum goes, but until then you rack up some serious damage. You also get a wall of 50 damage as a shield. I have no complaints about this cards power and life.
Air (Wyrm Queen): 8/16. 20 air quantams. The ability is great. I have nothing against it. It makes mono-air actually put up a fight. The power and life are a different story. I'd suggest it being 12/9. That's 3 times an Elite Wyrm, so that's more than fair.
Aether (Immortal King): 8/1. 20 aether quantams. I'd suggest it cost one more aether quantam for the ability. Other than that... no complaints.
All of these cards were good on your aspect, but they would all need different names. I'm sure you just popped out an easy name for examples, but the name needs to fit Elements more. Elements doesn't really have humans... so the names need to be different. Also, I'm kind of debating whether they should all be 20 quantams to cast or if they should vary. To make it more balanced of course.
Now to address the name. This game is Elements... heroes don't really make since. Heroes are a more human aspect and they're aren't really any humans in here. I think a better name would be Avatars. This goes a lot better with the flow of the game. It also makes sense.
