I have some experience in playing this sort of game in the ero variety, I can see the rudimentals of that sort of game here already so I'll comment on my experience playing through it.
Witchgirl goes a little schizo if you try to strafe and fire at the same time, looking left and right rapidly. Its cute, probably unintentional.
Witchgirl is super basic to the point that I am much happier having a secondary skill than her fireball, because I have to aim and hit and then collect the gem. That's a lot of processes. There could be more interesting upgrades like splitting her fireball into three and shooting in a cone rather than a bigger fireball upgrade to cover for this.
Bombergirl is intersting, I don't see this very often and she feels the exact reverse of Witchgirl, its quite fun to play because the answer to upgrades is always MORE BOMBS and pickup range. I was unsure if +fire added more damage to the bombs until I started stacking it. I get the feeling you can kick bombs, but I never got to pulling it off since I was always laying bombs anyway.
Summoner boss basically asks: Do you have enough AOE? If so, you can strafe him in circles until his summons > your pickup range. Its a good idea that his summons drop gems, though perhaps he could do with a cooldown of sorts as a first boss, like he gets tired after X summons and gives you a window to attack. There's also quite a bit of lag time before summoner finds you, perhaps spawn him a little nearer to the player?
You always get 10k gold when going in and out of the menu, so I tried unlocking the undeveloped characters (laugh). I should also note that it doesn't save your gold progress, which is understandable.
Good luck DungeonWatcherdev!