One thing to remember about the dungeons is that there will be THREE of the same battle in there. So if the dungeon’s preview page says that there will be three cards to fight, that means there will be nine of them, three of each. So make sure that your cards are far more powerful than the cards that you will be facing when you go into the dungeon.
Additionally, make sure that you are prepared to spend at least one Dragon Stone in order to restart a dungeon after you get beaten. It costs one Dragon Stone to revive in full, and these dungeons are so long that it’s sometimes an inevitability that you’ll end up spending one in order to get through the last of the battles. If you can’t spare even just one Dragon Stone, then complete a quest first, or power up a LOT before you go in.
The way to ensure that you win in the arena is to compare your cards’ rarity and level to that of the cards that you will be playing against. If they are stronger and more rare, then you will have a slim to none chance of winning. If they are not as strong and they are less rare, then you have a much better shot of winning, especially if they are at lower upgrade/power up levels than your cards are.
When you open up the helper card screen before a battle, if you don’t like anything that you see, close it and then open it back up again. When you open it again, a whole new bunch of helper cards will show up. Do this as many times as you need to in order to find a badass helper card, especially one that is 4 stars or even 5 stars if you are patient enough. Three star leaders are common. Five star leaders are RARE.
Keep ranking up until you unlock the ability to add extra card slots to your creatures. Once you do, you will be able to swap cards in and out for even more customization. If your creature has a card in its extra slot, don’t sell it or sacrifice it until you remove that extra card.
If you want to add more card points so that you can add rarer cards to your party, rank up, as you will earn one battle cost point per player level that you gain. However, if you want to add more card slots so that you can hold more, you will have to pay a Dragon Stone. Better instead to put your unwanted cards to use, such as selling or using them as power-up fodder.