Interesting game! I like that the characters and weapons give different buffs! I think it would be nice to have a little more feedback when you get hit (besides the hearts draining) and maybe a running count of orbs collected while playing. It might be interesting to go for a Loop Hero kind of thing where you can use the orbs to build and kind of grow your orb harvesting empire the more you play but you mentioned you didn't get levelling done in time so maybe you had something like that in mind.
I did notice quite a bit of slow down as more enemies started spawning. If you're using physics objects (area/kinematicbody etc) and you don't need the enemies to collide with each other, it might be worth putting them on their own layer and have them only collide with the player layer mask, might get you more frames.
Really nice game!