THEN LEVEL YOUR SKILLS!
One of the most absurd things to me is people who complain about how weak their decks are and they never level the skills on their cards. A player will spend a hundred HP on a new final form card that increases their deck from 80k to 82k but they won't increase their lead card to skill 10 which would be an 8k bonus on an 80k deck.
Especially now with the changes to skills its more important than ever to level the skills on your cards since leveling them also increases the chance to activate. Most high level fights are decided on who gets more or better procs than the opponent, not by a 2k difference in card quality.
Here's simple example. Five skill 10 Nightmares, an outdated card vs lets say five skill 1 Dragonewt. (For this example we're going to ignore the fact that the chance to proc 2 is reduced since they are the same card, just like I'm ignoring the fact that you would normally use two Trow wings with either of these)
Nightmares go from a 75k base to 92k if one procs.
Dragonewts go from 87k base to 93k if one procs.
Nightmares go from a 75k base to 109k if two proc.
Dragonewts go from 87k base to 99k if two proc.
Now, you can argue that Dragonewt only has an 8% buff and that's doing it. So lets play pretend. Lets pretend Dragonewt has a 13% buff like Nightmare.
Dragonewts go from 87k base to 98k if one procs.
Dragonewts go from 87k base to 109k if two proc.
So, Dragonewt would manage to tie up to Nightmare. A Nightmare deck that would have 12k less base stats and the best it could do is tie up with Nightmare at higher skill levels.
So remember, next time you're looking at spending your HP on that next level of card, take a look at your deck and the skill level of your cards. Chances are, increasing your cards to skill 10 will increase your deck power a lot more than just getting better cards.