This game is like a dream come true and I would have honestly loved it as a kid.
As an adult however I want to try and provide constructive criticism.
Even though the sprites looked nice, I just felt that the sizing between mons was very off, and in many levels I encountered problems with the hitboxes on both my characters, and the enemies that I faced against and it was very annoying how some mons just cannot hit some enemies at all, due to the sizing or hitbox problem.
There is no way to switch characters between levels, and that is just a shame, because the mechanics in the game make it so that different mons have advantages and disadvantages between the levels, such as how the Agumon line does well in the desert level as it doesn't receive heat damage, or how the Gomamon line does well in the water and ice level since it doesn't receive cold damage and can swim, and there are even mons that are not affected by the darkness since they can see in the dark, so I was just saddened as I found unlockable mons on my first playthrough with Agumon, but had to wait until I either cleared the game, or went back to the mode select screen but that just reset my progress in the stages, and I had to start from the first level again, even if it was nice that the unlockables and collectables were kept, so I can only recommend that the character switch shows up at the start of all levels to allow people to switch mons as they like.
I thought it was annoying how some boxes could just not be broken at all by some mons, even as they were on their ultimate or mega levels, and this made the arena more tedious, as the giant meat was always kept inside a box between the levels, and if your mon could not destroy the boxes, that means that you had to let yourself get killed between some bosses, as the only way to replenish your health, and I won't complain too much because at least you have infinite lives to do that, but it would be better if an update fixed it, and the recommendation would be, so that rookies need to use their projectile to smash the weakest box, while champions can melee the weakest box and will need to use their projectile for the crates, and then ultimates can melee the crates but will need their projectile for the metal crates, and then mega and above can break all boxes with their melee or projectiles.
One of the things that I did not like, was that once you evolved a mon, and you reverted the evolution on the mode select screen, you actually needed to collect more points to evolve it again, and I feel that is not only annoying but unnecessary, as you already spent points to unlock it, so I would recommend making it so that once you unlock an evolution, you can evolve or de-evolve as you like from the pause screen with no cost whatsoever, and this would actually make exploring the levels better, since for example Metal Greymon has double jumps and a longer reach with his metal arm, which makes for picking up certain collectibles possible, while if you just evolve him into War Greymon you lose the double jump as it gets replaced by a glide, and his Anti-Dramon claws don't have as much reach as the previous evolution metal arms.
Aside from what I mentioned I still find the game fun, and I would love to see it more polished and improved. And heck if someday a multiplayer co-op mode gets made, or even a PVP mode then that would just make it even better.