Hey mate, one of the things that define pixel art is controlled color use and for example in this piece I don't see any reason to use more than 30 colors but you have used 135 different colors, which means you've used slightly different tones of the same colors. For that and for some overall polish, I suggest you to go to our WIP forum and look for some advice.
I agree with everything you said, I was a bit tired of this piece actually but I will make those corrections later. I also want to improve the light in the dragon. Thank you for the feedback!
You wrote "then" instead of "than" xP
This is probably an odd thing to comment on, but I love the use of an outline to suggest bold text without actually making it take up more space.
I feel this piece could use a tidying pass, some of the curves look jagged (e.g. the attack and vitality shapes) and the card name looks a bit messy.
Thanks for the advice, I'm still learning about pixelart and I think the guilt of the large number of colors is due to the transparency I used around the character.