Good concept. Roughly decent execution.
The NPC movement speed is a bit too high- hard to dodge in time. Also, some kind of (visual) feedback would be helpful, as a warning that an NPC is headed towards the player.
The arena could be slightly bigger. I get that you want it to be small so that characters can fall, but I think that increasing it a little bit might reduce the risk of the player falling off- which I did more often than I got knocked off by the NPC.
It would be cool if there was a way to easily know how far away I am from the edge. Often, it is hard to see the shadows, and some times the ship fell down when I thought I was still on the floor. From the top of my mind, you could add some visual feedback for when the player is at the very edge.
I never survived long enough to try a power up or a portal. I managed to play up to the third level before giving up.
You may want to reduce the animation time at the start of each level. After selecting a level, I had to wait ten seconds to actually play. Generally, it would not matter if the level is loaded for the first time, but when the player needs to restart the level frequently, it gets annoying after a few times.
It's a nice concept and it has good potential that can be reached with further polish. Nice work!