Best cards V1:
Damage Dealers

Xayah is one of the most powerful cards in V1.
Her skill Chop will deal up to 160% of her ATK each 6 rounds. With the secondary skill, Smite, her ATK will go up to 24%. She can knock out easily cards in few turns.
If you want to make her REALLY powerful, her 3rd skill should be Snipe, that deals up to 150% ATK + 60 to a random enemy cards. This hurts a lot!

Amoura can be a very bad girl too.
Her first skill, Execution, deals up to 30% of the basic HP of the opponent cards (this is 1/3 of his HP!) each 8 turns.
But this is not all. Her second skill, Combo, can launch a second normal attack (60% probability) in a row! She can really kill a card if you're enough lucky.
If you want to go to a higher level, her 3rd skills are really destructive with Sacrifice, Chop or Defeat. Choose you favourite!

Kagura Is a strong support for her skill Shoot, that deals up to 60% damage to all your opponent cards each 8 rounds.
This skill can beat that enemy cards left with a few HP and make the job done.
It is ok to play with a Rare versione of her.
The second skill is not so strong but can deal more damage to new cards that your opponent plays, which is Weakness Aura.
Her 3rd skills are very strong too, but Kagura is very rare to find in Legendary/Myth version.

Luna is a very good card. The combination of her skills can deal a lot of damage.
First skill is Defeat: deals up to 130% of damage and reduce opponent ATK up to 15% for 2 rounds.
Her second skill, Ancestral Bloodline, increase her ATK by 5% each 5% HP loss.
This skill increases her basic ATK and the damage dealt by Defeat.
I suggest to support Luna with an Healer like Aurora, Celia or Sylph to increase constantly her ATK.

Cres is a brand new Hero. She have the potential to knock out your opponent with her skills: Defeat (Luna has too) and Crit.
Crit effect is really "critical": you have up to 42% of probability to deal x2.2 damage to your normal attack!

Eula is recent Hero, an ice archer that can hurt a lot. Her first skill is Chop (as Xayah), and her second skill is Multiple Attack: for each normal attack, she deal up to 60% of ATK to a random enemy card.
This second skill can really annoying your opponent and give the final blow to other cards with low HP!

Naomi has two very interesting skills: Bloodthirsty deals up to 130% ATK damage and restore 30% of the damage dealt to you HP. Very useful to make her survive more turns.
The second skills is Critical Hit: similar to Crit, there is a chance 25% chance to deal up to x3 damage to a normal attack! She can really knock out fast any enemy if you are enough lucky.