So thanks for making it look nice to play.
Doing some cards in pixel art is a great idea.
The colors and drawing look nice.
The timing and staging of the animations could be improved.
But assuming you have other things to do, it's visually readable.
Here's an edit to exemplify my super vague statement above.
The timing of the animation is a bit slow.
It works with the limited number of frames.
And will save you time.
But loses some of the impact and action potential of whats happening in the scene.
By staging I mean that the overall presentation of what we need to see and feel is not as strong as it could be.
The tree is assumed to be protecting the guy from the rain of fire.
But none of the fireballs strike the branch above him.
It's honestly easy to imagine that the tree is giving him cover.
But showing it to the viewer is a much more powerful form of composing of the animation.
Below are all the steps I made to reach the above edit.
Cleaned off the image.
Base rain loop. --> 4 frames
Small impacts from the rain placed on the ground and the branch. --> 16 frames
This turns the base rain loop into a quad loop.
Which is good, gives us variation.
Added sparks, more impacts.
Carved out the base rain loop with an overlapping ground layer.
This is a bit messy and 2 dimensional, but a good quick fix.
Animated a large fireball.
Then scrolled it over to the right to add another and save time.
Added a color flash in response to the impact of a large fireball.
Was done in index mode, so no colors were added if that matters to you.
Composited all the files together.
A full loop without any large fireballs was added to the middle to give them some space.
I thought a light screenshake for the large fireball impacts would be nice.
Then I added a messy fizzling fire to help resolve the large impacts.
Again just scrolled over the same animation to save time.
Hope that gives you some ideas.
I would say similar things about the other animation.
They both look really good, they just don't have a lot of feeling in the motion.
Timing and staging go hand in hand.
They have great affect on each other and messing with them will help to provide technical challenges and potentially more dramatic presentation/response.
So play around and see what you can do.