Hi Gouenji! I'm also relatively new here, but it looks like you'll be fine. Here are the main things that I noticed:
1: The pants are baggy and the arms are slightly short. This is a problem because it's harder to see exactly the fluid movement you put into the animation, and until you are confident with other sticks, should you use something harder to see and express, and also makes it look less powerful (like LordPivot said)
2: The screen shakes when he makes the fireball, but it doesn't really shake when he stomps on the ground or when the fireball explodes. You therefore are belittling the stomp and explosion, which in my eyes should be the exact opposite. Nice work on the special effects though!
3: To give it a more lifelike stomp, you should have your entire body more pressed against the ground, rather than mainly just his leg, (eg. his back could be tilted more down, his arms thrown back).
4: Before the guy jumps into the air, he should crouch more, to seem like he is actually pushing himself into the air more powerfully, (unless he can fly, then you can ignore this one
).
5: To make the hole left on the ground seem more realistic, you should have it approximately the same size and shape as the rock that came out of it, with the exception of small cracks, and small bits of rock.
6: The flying kick seems very weak, partly because he just seems to fling his legs out in both sides. First off, that wouldn't make sense considering the fact that there is nothing on the other side, and second, you should have him just make a slightly more exaggerated kick, like pull back, and slam his foot into the rock, breaking it.
7: To put more suspense into your kick in the sky, you can have them rise a bit more slowly, and right after they reach the peak of how high they go, then your character can kick it.
8: When your character lands after kicking with both feet in the air (in the beginning), you should make sure his feet/legs take the recoil (as i call it). The "recoil" is after a person lands. When the person lands, you should have his feet bend down more, indicating that there is gravity, and that you can't just kick into the air and land, light as a feather.
All and all, beautiful execution, and the smoothness is close to perfection. Keep it up!