I thought this might be an endless game, but then a boss appeared!
Very nice, if simple game. :)
I liked that the slimes could speed up to add some diversity and, of course, the big boss at the end. The animation for the bombs was also satisfying.
Speaking of the bomb, while it has the weakness of a shorter range compared to the main attack, I found it was still far easier to use it exclusively. It destroys everything that it hits, meaning that the slimes which are further out are simply waiting their turn to be exploded when they get within range. For a more balanced approach you could consider limiting the number of bombs, or putting them on a higher cooldown, so that the player has to rely on their main attack some (or most) of the time.
Other than that, sound and music always help. :)